If the word Trailhead means the place where you stop to air down, then you can skip to the GADGETS section. If the thought of deflating your tires below manufacturer specifications sounds crazy, read WHY first.
Deflating tires from street pressure to trail pressure is a long accepted and highly recommended practice. The key to successful off-roading is traction. The most common off-road oriented modifications - Tires and Suspension, do one thing for off-road performance - improve traction. For those who haven't upgraded those items, and for those who have, airing down for the trail is just as valuable. Simply put, lower pressure gives you a bigger foot print and this equates to more traction. As an environmental bonus, since the bigger footprint with more traction is less likely to slip, less soil erosion is induced.
HOW TO AIR DOWN
Deflating is the easy part. To do it with some degree of accuracy, you need a tire gauge. Generally speaking, all four tires get the same pressure. To do this effectively, use only one gauge. Low priced gauges aren't highly accurate to begin with, so when you are trying to balance the tire pressures with in 1 PSI, two different gauges confounds the process. You've got four tires to do and it takes a little time. The old trick of loosening the valve core will work but I don't advise it because the hassle of breaking or losing this small part is not worth the few minutes you might save. To help speed up the process though, there are a few 'hands-free' deflation tools available. When using one of these 'hands-free' air bleeders, work one side of the vehicle at a time and check the pressure frequently. It's a little silly to have to air UP before hitting the trail. Be conservative. It is safer and easier to stop and let out another pound or two than to run the risk of driving on a "flat" tire and having to air up. This is a good practice anyway since proper off-road inflation is so variable.
I'm the proud owner of the most 'gadgety' air device you can buy, the Quadra-Flate by Quadra-Flate Enterprises. It's the four-tentacled octopus of air gauges. They are available in small, medium and large sizes (hose lengths). The Grand Cherokee is a medium. There are four air lines, each with a clip-on air chuck at each end and a quality dial gauge in the middle with a bleeder/inlet valve. The first step is to attach the gauge / valve to the lift gate striker with a Velcro-type strap. Making sure the valve is closed. This holds it in a convenient, visible location and keeps it out of the dirt. Clip a chuck to each tire valve stem. Open the bleed valve and you're in business. It may take less than a minute to reach your target pressure. You must close the valve to get an accurate gauge reading. When you are finished, with the valve still closed, unclip each chuck, unstrap and stow. This device works even better for airing up because it balances pressure across all tires. The convenience factor is that you can clip on your pump and relax for a while. Quadra-Flate's claim to fame is that it enables balanced pressure in all four tires instantly. The Jeep Owner Manual strongly emphasizes the importance of balanced inflation on Quadratrak equipped vehicles.
Another handy item from Sun Performance Productsis the QuickAIR Deflator (set). Simply screw on to the valve stem and check the pressure with the unit in place. Since you don't have to hold the valve open, you can do more than one at a time. My advice is to do only two at a time, on the same side of the vehicle because you have to keep checking with a hand held gauge. The rate of airflow is adjusted by how far you screw the adapter on. You could set one at a fast rate and the other slower, monitor the fast one as a primary; when you're done, sew it up and leap frog around while you finish number two. Why not make a game of it! They come in a set of four, and screwed together, are no bigger than a pen. That has some value for us space-conscience Jeepers.
A fairly new item on the deflation device market is the Trailhead Automatic Tire Deflators, from Oasis Off Road Mfg., which deflates automatically to a preset pressure. If this device is accurate and reliable, it may be the most convenient of all for the deflation part of Air Care.
HOW TO AIR UP
Kinda like going down hill, deflation is the easy part. The flip side, inflating, is just as important to paved-roading. There are two basic ways to accomplish this, a container of compressed air or a compressor. In case you think I forgot about hand and foot pumps, you're right, I forgot about those a long time ago. For about $20 on sale at K-Mart, you can pick up a 5-gallon air tank. For that price, you'd better get two because one ain't enough. As a point of reference, one 5-gallon air tank compressed to about 115 PSI will deliver about 5 PSI increase to four 30X9.5 tires. Of course a pair of these tanks starts taking up some serious cargo space because they are awkwardly shaped. Clever fabricators have turned bumpers and even roll cages into air tanks, but there are practically no such applications for Grand Cherokees.
Vendors like Currie Enterprises have vehicle mounted combo kits, that is: compressor, tank and related hardware such as gauges, hoses, regulator, electrical connections and brackets. This is the trick setup, professional, expensive and very hard to apply to the ZJ. It's that available space thing again. While you may consider it an option, I count very few votes for permanently mounting a compressor or tank in the passenger compartment. Still, this combination is the best concept. A tank provides fast flow at high volume, but lacks sufficient capacity. So a compressor is still required.
A quality compressor such as the Quick Air and the Quick Air II by Sun Performance Products combined with a portable tank is a good alternative. The performance is near that of the expensive kits and you don't have to live with it every day
The new kid on the inflation block is the Power Tank by Advanced Air Systems, Inc. CO2 is nothing new, but its application to the off-road sport is still novel. Its advantage is that its compressibility in the proper tank allows for the filling of many large tires. This concept is still new enough that a comfort level among the off-roading populace has yet to develop about questions like - is it OK to put CO2 in my tires, and is it safe to carry such a highly pressurized vessel in or on my vehicle?
Does it seem like there's a lot of choices here? It should because that's how big a deal the Air Care issue is. Most of us make our equipment decisions based on budget, but as for priority, this stuff ranks right up there with the shovel and tow strap.