Rear end suggestions

Yes. The joints are sometimes installed too tight. You can remove the driveshaft and flop the joints to make sure they move freely. But I think the problem is in the motor. Like a vacuum leak causing a lean condition to one cylinder or a bad plug wire or plug, or carbon buildup in one cylinder causing pre-ignition. An imbalance problem ALWAYS becomes more pronounced with the rpm of the part.
Engine was just rebuilt and still being broken in. 210ish miles since been installed in the car. Plug wires are new (doesn’t mean one still can’t be bad tho).