I'm thinking of taking mine out and getting it checked for balance and to make sure it isn't too long now. With it being lowered and shortening the distance between the yokes maybe that is what causing the vibration. I'm sure if I take a good measurement between the yokes the driveline company can tell me what an ideal length is.

 

I had the same problem. Make sure you adjust it ASAP.. I didnt fix it in time and my bearings went out in the rear end..had to replace all that. i added shims or pinion angle wedges on mine and its running fine...also make sure the U bolts are tight..might want to put two nuts back to back so it does not loosen up while driving it hard. There's another thread in here which helped me out as well I dont recall the name tho.