I did notice that my drive shaft is pushed into the tail housing almost all the way to the yoke. Could this be the problem?
Sounds like your problem area, whether the shaft worked with the automatic, is a mute point now, you need some (3/4"-1") Slip play to allow the shaft/spline movement as the rear suspension compresses over it's travel,....I'd suspect you have a Dodge drive shaft in your Plymouth, as they are different,....measure the shaft center to center, here's the specs
68-9 Dodge B body BBlk 4spd w/ 8 3/4......52 1/16"
68-9 Dodge B body BBLK 4spd w/ 9 3/4......50.960"
68-9 Plymouth B body BBlk 4spd w/ 8 3/4...51.500"
69-9 Plymouth B Body BBlk 4spd w/ 9 3/4...50.390"
