If you have ruled out the rear diff, then I'd swing down to your local auto parts store and spend the $20 (total) and put some new universals on and check for rumble/vibration again. If you still have the issue, take the shaft down to the local driveline shop and have it examined and balanced. They'd likely put new u-joints on to balance it anyhow.
Did you say that the PO disassembled the shaft for towing? Usually folks just disconnect both ends from the diff/transfer case respectively, in which case I don't think it matters which way you put it back on as it won't affect the balance of the system. Yoinking the slip joint apart on the other hands could be a problem...
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).