I have an 1998 ml320 and for the life of me I can not figure out how to get the dead battery out of it to change it. It seems to be in some sort of battery tray cage. PLEASE HELP!!

It's a little tricky, not too bad.
First, disconnect the rear (negative) battery cable, then disconnect the front one (positive). The positive cable also has a smaller cable secured to it, I usually remove that one too.
Then remove the rubber strip on top of the metal wall and plastic filler piece-it just pulls off.
Pull the positive cable out of the plastic filler piece, by first unsnapping the small "door" on top of the cable on the filler piece, you may need to pry it open with a screwdriver. The plastic filler piece can now be unsnapped and removed.
Once this is all done, you will notice a bolt with a 13mm head at the front of the metal wall that surrounds the battery. Remove that bolt. You are "almost" there!
You now need to remove that funny looking black clip on top of the battery, the one under the metal strap. When that's removed, you can push down on the strap and unhook the strap at the bottom of the battery tray, the side towards the fender. There is a similar hook on the side of the metal wall facing the engine. Undo that side and the entire wall and strap can be pivoted forward, away from the firewall and the 2 tabs on the front (near where the bolt was) can be disengaged. If you want to remove the wall and strap assembly completely, there is a water pump mounted to the inside of the wall which can simply be pulled up off it's bracket and left lay there, but be sure to put it back on its bracket when you're through.

Gilly