put in 8 3/4, now have vibration

any suggestions on what might be the most likely culprit?

My car is 71 demon with 360 and 4spd OD tranny. I just put in 8 3/4 used carrier 741. Its 3.91 gear with suregrip.

Suregrip works good, noise is minimal, however it starts a very fine vibration close to 60mph then gets steadily worse as speed goes up from there.

I got new big bolt pattern axles with green bearings from Dr. Diff and everything installed good.

Note: the used chunk I got showed some corrosion on several teeth of the big gear but I figured it wouldn't affect anything...