rhythmic vibration on hiway

Yup. You cannot check a shaft without pulling it out. Remove the two rear caps and inspect for grease. "Easily" move the other two bearings, feeling for "smooth movement" should not fee dry or "crunchy" or overly tight

Now wiggle front joint in every possible way, same thing.

Check pinion bearing/ drive yoke for looseness. Rotate rear yoke back, forth, feeling for excessive play/ wear, and side looseness. Don't discount rear may just be low on lube

You may be developing serious ring/ pinion wear problems.