90 SC Trans Shuddering when stopping

I think I'm in trouble. I had a local shop (with a good reputation) rebuild my AOD. It had a very hard 3-4 shift and after shifting into 4th would shudder when I came to a stop. This would only happen if I alowed the car to shift to 4th. In less than a week the trans lost 4th gear alltogeather (felt like it was shifting from 3erd to neutral). As it was under warrenty, the shop had a tow truck come and get the car and built a new trans for it. He said the old trans was toast so he dumped the whole thing and built a new one from scratch. This one is doing the same thing. The 3-4 shift absolutely slams into gear at lite throttle and when I come to a stop it feels like the trans is locked up. If I drop the shifter into low when the shuddering starts, it will stop the shuddering instantly. I am taking it back to the shop but would appreciate any thoughts you might have. Oh, it has a new converter and valve body as well. The transmission guy seems to be a great guy and his warranty is good as gold but I've paid almost $2200 for the trans and I don't think it's too much to ask that it be right.