Not sure what parts your referring to?? If the main shaft of the transmission (shaft that both spurs and clutch ride on) is moving, there'sprobably  a setscrew loose somewhere.. Most likely on one/or both of the drive cups.. OR  If the dogbone is moving back and not spinning the drive cups, you can shim it a little by putting an "o" ring (or little piece of fuel line) in the opposite drive cup, to keep it from moving back and forth.. (The dogbone slipping is unlikely, cause if it slides back to the point where it doesnt spin the drive cup, it'll just fall out completely and you'd loose it.) Pull the tranny and make sure the setscrews are tight, and tightening up on the flat part of the shaft..