Sounds exactly like the problem I had after I rebuilt my transmission.
It spun the wheels with them off ground, backed up okay on the ground, then wouldn't drive forward and felt like it was loading up and binding internally, and wouldn't move forward.
When I stripped it apart I found I had mistakenly installed the rear clutch piston backwards.
Just a shot in the dark. There are so many things that it can be causing your problem, but it sounds alot like the issue I had.