I have a 1993 Mazda MPV with 4WD with a tranny problem

My 1993 Mazda MPV's transmission. I replaced the transmission with one that I had taken out of my old MPV as it worked fine. I bout this MPV with a bad tranny. I changed the tranny and it worked fine for about 3mnths with no problems. One day I took it out to drop off a friend and after about 6 or 7 miles I turned onto a road and the vehicle down shifted like normal then all of a sudden it stopped moving at all. There were no noises or slipping and grabbing, it just would not move since then. Is it possible that it is electrical or can a plugged cooler line do that as I did not replace the cooler. It just seems very strange that it had no previous symptoms and it happened all of a sudden. I tried in vain to change the fluid and filter but it did no good. As a matter of fact if you put it in gear the rpms actually rise a little rather than going down. I need help, thank you in advance. I really enjoy the vehicle and would like to have it running. Tab