Best to take a hose, or the wand at a car wash, and hit every hole in the sides of the frame that you can. Water will start running out of other holes in the frame when you do this. I'd do it until what comes out is clear. Some, including myself, have drilled drain holes in the bottom of the frame, right behind the front control arm mounts as well as right in front of the rear control arm mounts. If I remember correctly, I drilled 3/8" holes, or somewhere near that size. touch up the holes with some paint to reduce rust.
On the sides o the frame, near the back of the front tires are big openings. I got some plugs and put in there to keep the front tires from throwing as much stuff in there. There are holes on the opposite side on the inside, but I didn't plug those, just the outboard ones. That alone will not eliminate getting stuff in your frame, but it will help some.
If you have one of those fan spray sprinklers, it will help clean the underside some. I do that after making trips to the Outer Banks and driving on the beach. Just move it around in 4 or 5 places for about 15 minutes each.
By the looks of your jeep, you're going to have a lot up mud under it to clean out/off.
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