Have new Ujoints in and have the drive line balanced. Check your pinion angle with an angle finder while the car is on the ground may have to shim the axel housing...
How does the 904 sound when is in neutral, and in-gear stopped? If your not hearing a bunch of wheining, or fluid pressure ramps, shifts smooth but strong and trans fluid does not smell like death, your trans is mostly fine. Thay are super easy to freshen up...