I'm not sure yet, or even if it's the axle. At speed there's a lot of noise coming out of the rear end. If you do about 70mph it gets pretty loud, if I get off the gas it goes away. Also, if you alternate coasting, and then applying a little gas, you can hear gear noises in the rear. I think it's a noise when the ring gear is going from the coast side to the drive side.
There's also a clicking noise when coasting, but that could be something else (tcase/tranny).
I don't know if it can be adjusted, I know these 8.25's have threaded adjusters for backlash.
Not sure if the shorter driveshaft would help, I put my brother's in the jeep and it made the same noises/vibes as mine. Of course his is the same length, so it really just eliminated my ujoints.