Mopar Forums - tranny woes

Hi guys . Yes I,m a newbee. & I've got a transmission prob.
Well I bought a parts truck , just to get few small part I needed for my 85 W150 project. The parts trk is a 88 D100 long wheel base 2 Wd which had a bad rear end. The rest of the trk is in great shape. Too good for a parts trk. So I bought another parts trk (84 D 150) for my parts trk.
Well with that said When I went to get the D100 I started it for the first time in about a year & tried to pull onto my buddy"s trailer It wouldn't go because the rear was too bad. The trk would attempt to pull itself onto the trailer. It would spin or the rear would make rock crushing noises & slip. The tranny seemed fine. After getting it home via rollback it set for a week. When I went to pull it in the shop to swap rears The trans was slipping in forward gear. after I completed the rear swap it is still slipping. In drive it tries to move. in low it will move but slips when you accelerate once you get moving enough to shift into second everything works fine, up shifts, down shifts good until you get slow enough to go back into low than its slipping again.
Is there a quick fix or do I need a nother parts truck? Unsure what the tranny is at this time. It in not an over drive. 88 D100