I started with stock aluminum and switched to aftermarket steel with the 9 inch. The steel was quite a bit worse but I still got mine vibration free with the steel shaft. Since then I switched to an aftermarket aluminum unit.

 

I have been hunting down this vibration as well. I got a longer slip yoke and it helped a little as I also have a 9" bout 1" longer. I have a lesser vibration at 70-80mph but it's still not gone. My u-joint didnt flop around like mine use to. I thought if it flopped around it was fine? It's suppose to be that loose. I'll replace it when i get my other vehicle running next week.

 

I replaced my U joint and my gears on my Moser. Went with the same ratio 4.11 as before. Vibration is gone but not completely, next im going to replace the diff. Piece by piece im buying myself a new damn rear end.....after buying a new rear end.