Get the rear wheels off the ground, add the TransX, start it and run it in reverse for a while, then run it forward. If it stops, put it back in reverse. There could be some schmutz in the valve body, or maybe some pump/governor issues.

The B2 piston engages brake band 2 when the car is in 1st thru 3rd. In 4th it is disengaged and clutch K2 is engaged. When this piston fails the car usually won't go forward at all. The B1 piston is only used for 2nd gear. Reverse uses the B3 brake (which looks like a clutch pack to me, oh well).

What is interesting is that it stayed in reverse, makes me think there are valve body issues or some kind of strange pressure issues. Does the fluid look ok?