[2021STK]

One odd thing to check, the passenger side motor mount to the frame. Many moons ago I kept breaking TH400 bells, car had solid mounts, turn buckle on drivers side, rubbery trans mount. Finally one day while I was doing dry hops in the drive way with the hood off, everything looked great until I backed up and the engine raised up about 20* off the frame. I found out the original motor mounts on the frame had torn off the welds under the mounts. I had checked all the attaching bolts first for being tight but the frame metal was torn out of the crossmember on the passenger side. After replated the frame and adding turn buckle never had another trans issue.

Good luck finding your problem