There should be two rubber bumper inserts, one on either end of the drive shaft. If one is missing, the splines might not be fully engaged thus will slip and wear prematurely.
A 2nd possibility is engine alignment may be off too much, confirm using the engine alignment fixture.
3rd thing to consider are the engine mounts. If one is broken lose the engine will tilt due to torque and the driveline will operate at an angle, wearing the splines.
Let us know what you discover.