Is it an automatic? If it is, its the a4ld. My 94 4x4 explorer did that last week and this isnt a positive thing, I just took the trans out today. Kept messing with it and I thought it was the front end and then I removed the transfer case. The transmission was locking up in reverse. Sometimes I could make it go backwards but it would lock up and act like it was in park. I was told it was a spring inside the transmission that is supposed to let the transmission go forward or backwards, but it locks up and will only let it go in forward. Thats exactly what mine did. Was told it was a losing battle, was told that there was nothing to do but to replace it or get that fixed, but either way, its all about the same price. Hope it works out for you, cause it definitely didnt for me.