Oh man, sorry to hear it didn't fix your problem. Can you jack the rear and let it spin? Maybe you can spot/feel/hear some anomaly. What about wheels or tires? Not uncommon to get a tire or rim damaged and cause vibration.
As for the spline end getting warm, I would guess that's completely normal. After all, you still have a lot of metal to metal spinning at high rates. Just about any part of the drivetrain gets hot when you drive.
we've jacked it up and spun the wheels, things look ok but still vibration. Tires are new, wheels seems straight and we've moved them around to all positions with no change. Also drive the wheels in the air without the tires and wheels, still vibration.