Finally got around to where I could see the problem. Part #50 (anti-rotate bracket) was cracked and only being held together by a thread. It wasn't bent but actually cracked at the bend and became more of a hinge than a bracket. So it didn't fully break appart but flexed enough to allow the tranny to flex. After I removed the two large lug type wheels I could also see where the right side trans / axle mount part of the frame was also badly broken. I was moving fast in reverse backing into the woods so I could move a large snow bank and hit a tree stump hidden in several inches of snow. Had to completely dissasemble the whole back end AND also found the transmission case cracked as well. I can't believe the whole thing was being held together at all.
Sorry to not get back to the replies quicker. Spent a few days repairing and modifying the frame and am waiting for the new bracket and transmission to arrive.