From personal experience, the only real way to isolate expensive noises from the final drive area is to pull the bevel drive from the swingarm and try it by turning it by hand. If it feels ok, pull the swingarm and UJ to expose the gearbox O/P flange.
If the gearbox flange turns smoothly and evenly, you're laughing. If the UJ is excessively notchy, there's your problem.
I've found a couple times on bikes, a couple things can be going south and the sounds manifest in several places along the drive. Using a stethescope in these instances can be misleading. Whatever the problem, gravel sounds there can't be good.
BTW, the driveshaft can run with minimal oil without too much problem, though obviously It's good to keep topped up.
Bill.......................;-)