Grinding/growling noise coming from rear end

[SquareOne]

Redland, just because the u-joints didn't appear to have slack, doesn't mean there isn't an issue. I've see where one cap is dried out, making the whole shaft shake. I've also changed a set just recently (about 450k miles) which felt fine, but when the caps were removed, you could see the needle bearings etching into the U-joint shaft itself, thus causing a slight vibration.

Check your slip-yokes too if you haven't done so (as in actually pull them apart and verify the coating is still present on the splines). Depending on mileage, maintenance frequency, and use, these can certainly have you chasing your tail at higher speeds.

I'd try to rule out the possible easier repairs before going too deep...