U-Joint Question - Part 2

Hey Folks,
As noted in a previous thread, I developed a vibration that only manifests itself when accelerating/under load. Was testing a new tune, hammered it hard in first, hit 2nd very solid at 6k, hooked great with no wheel spin. Coasted to a stop. Then noticed a subtle vibration when accelerating up to 60mph. Most pronounced between 45 and 65 mph.

I figured, u joints were the culprit. This morning I got the car up on stands to pull em and measure for replacements. I was expecting to find some play and/or visible damage, but other than being filthy and caked we crud, they appear tight, rubber in good shape etc...

So my question, has anyone experienced a problem witb u-joints that showed no visible damage? Would bad bearing caps cause similar symptoms? Also, hearing a "chirp" that sounds like a loose belt when letting off in 1st (sometimes in 2nd) after vigorous blips. Belt is tight.

Thought I'd ask members with more experience than me before digging into the ujoints.

Car is a 72 Duster 340, 727 w RMVB, 742 rearend.

Thanks as always,
Pat