You'll have to learn the places mud likes to hide on the X. One of the worst is on the bottom outside of the rear wheel openings, in front of and behind the tire. A lot of crap will accumulate there and if left it's just asking to rust things out. Heck, there's gonna be a lot of mud all through the wheel wells, including up high behind the lip moulding, I spend 2-3 minutes on each alone. Also a good idea to remove the lip mouldings now and then to clear out anything stuck behind them.
Of course pay attention to the front suspension, front and rear, lots of nooks for mud to build up, as well as the backs of the wheels/brakes, but try not to direct the full-pressure spray directly on the area where the CVs meet the wheel hub. Top of the leaf springs, tops of the crossmembers around the transmission and transfer case, the gap between the gas tank and skid plate, as well as the top of the tank, and oh yeah, a couple pounds always ends up on top of the back of the front skidplate/splash guard
After getting more than a bit muddy, I find it takes a good $15 and 20-30 minutes at the D.I.Y. high-pressure carwash to really do a good job of getting all the mud out from under the X.
Also, budget an extra $2-3 to wash down the bay when you're done, they really hate people who leave a few pounds of mud, sand, and pebbles.
Another option is to get a good old-fashioned sweep/arc sprinkler, position it under the X sweeping side to side and let it run under the front a few minutes, move it to the middle a few minutes, then the rear a few minutes, then do it all over again with it turned 90 degrees to sweep front to back. This will loosen everything up pretty well and then you can go after it with a garden hose to knock it out.
Brent