why does my jeep grand wagoneer keep dropping out of 2 wheel drive when it does that i cannot shift into 4 wheel drive or anything is my transfer case bad or do i have a vaccume leak

What do you mean "dropping out of 2wd" and "can't shift into 4wd", does it stop pulling itself? These Jeeps have vacuum actuated 4wd system for the most part. The lever under the dash, above the gas pedal is vacuum operated,(you should be able to move it freely as it just goes to engine vacuum), then hoses run down to a vacuum pod with a lever attached to it that pushes and pulls a lever on the transfer case shifting it from 2wd to 4wd, however there is no neutral there. It is either in 2wd or 4wd, it may get stuck and pull the lever just enough to start 4wd but not completely cause a grinding noise if the front and rear tires spin at different speeds like your stuck. Next there is a lever on the floor on the right side under the driver seat, this lever is to change from high (normal) to low range when in 4wd, you must be in 4wd and the transmission in neutral, and be moving 3-5mph to be able to move this lever into low range. The transmission must be in neutral before moving this lever at all. Let me know if that help or if not, expain in more detail for me. I am a Jeep GW expert lol, no really check out my garage here.

Very helpful Thank you.