Stephany Hayes

My 94 chevy 1500 4×4 will not move. I put tranny fluid in it numerous times and it would kick in but not for very long. Theres never a puddle of fluid under it so it cant be leaking out. It sat for a month out of my sight and control. When i got it back it wasnt right. The 4 wheel drive doesnt light up on the floor or kick in anymore. I feel like ive been dooped and someone has swapped rearends or somethinf. So before i start tearimg my stuff apart what should i check? Also what years are interchangeable with the rearend?