96 GA - shifts VERY hard into drive when engine cool

I have a problem with my car shifting very hard into gear. If the rpms are anywhere above 500 it will shift hard, it usually starts cold at around 1200 rpm. I can't figure it out. Any ideas???
You should also know that it only started happening since I had the tranny replaced with a used one, and no, I can not take it back to the place that I had it installed unfortunately.

I hope someone has some advice!

Thanks

Jay
(96 Grand Am SE)