I would loosen the rear u-joint and the see if there is any play between the locating tabs in the yoke. I had a slight annoying vibration also. I had 0.012" play by pushing and pulling the joint against the tabs
As a temporary fix I cut two pieces of 0.006" feeler gauge to fit between the two tabs.
Looks like you have a "long" yoke, very hard to find in a 1310 u-joint size. You have to get new or you risk having the same problem. I got one at Quick Performance, only thing it is a 1330 u-joint. You have to use a conversion joint 1310 by 1330
If the above is good I would have a driveshaft shop check the balance