This might sound very stupid but I had a buddy with a similar issue on his grand am. I kept looking around to figure out wtf was going on and come to find out the tech who installed his transmission forgot to tighten down the torque converter bolts so they flew out over time causing him lose all shifting except reverse. I'm not saying that this is your issue but just take a quick look at those bolts lol
That would be the dumnbest thing he could do,but it would make me feel like a kid in a candy store if that amongst any other small thing such as the filter,pnp switch (not saying the switch would do that) etc,etc.
'01 GTP: Rebuilt Trans by Andrew S....300m,hardened 4th,Transgo shift kit,V8 chain ect ect,3.8 MPS,ZZP 10.5.....More to come!