...Would that point to bad U-joints? Because I couldn't get it to budge, which is a good thing, right.

Here's what I did, I jacked up the rear of the car with the E-brake on of course, car off and in park, I reached up and tried with all my might to rotate the Driveshaft. It wouldn't budge. So my U-joints are fine right?

Please someone tell me that I am correct and to look somewhere else for the cause of my vibrations.

 

You could also take the shaft out and check them for binding.

 

I would say that you're correct in the U/J's being fine. If they were worn then you would be able to rotate the driveshaft (propshaft).
However, it may be worth unbolting the rear u/j clamps, rotating the propshaft then re-clamping so that the rear u/j is now 180 degrees out from its original position in the diff. yoke

Turns out that my T/A had the u/j fitted wrong - once I'd done that, the driveline vibrations went away.

Mark.