So they basically swapped in new wear parts, but never took the time to actually diagnose why the transmission wouldn't shift....:thinking: I'm no professional mechanic, but that seems to me like a shady way of "patching" the problem....

Since they failed to solve the problem at all, I'd go back and tell them to get it taken care of... For $2000, they should be able to diagnose the problem and take decisive action to repair it, not use a "shotgun" approach like the one they've used.

To be frank, I do not have a very good impression of these "mechanics". They sound shady to me, based on what you've told me..