Just an FYI to clarify a couple of points.
There is only one (1) universal joint in a Shadow shaft drive system. It's up by the swing arm pivot.
There is no oil in the drive shaft or U joint space. The oil is in and around the ring and pinion gears in the rear wheel hub.
Without hearing the noise, I have to say it could be any number of things. Brakes doing something bad, wheel bearings, shocks, rear end gears, hub pins, swing arm bearings, there is probably something else but that's all I can think of at the moment.
Start with the easiest to check first. Gear oil, brakes, bearings, and then U joint. Getting at the U joint, while not extremely difficult, is a pain in the azz.