Shackles = vibration for me also. I installed 2" drop shackles and immediately had a vibration above 72 mph. I tried 2 degree shims, though no shims are supposed to be required with 2" shackles, to no avail. I have since removed the drop shackles and installed new stock shackles but still have the freaking vibration. I had the driveshaft checked for balance and replaced the U-joints also. I am thinking of getting a second opinion on the balance.

 

Not to worry, they were broken in plenty before this, it was too much of a headache to go and screw the whole thing up. I use a taper shim from a old leaf pack, it was prob close to 1'' on the taller side

 

its the drive shaft bro i am having the same issue with my crew cab even before it was lowered it would vibrate sone as u got 90to about 105 and it would smooth out and when i lowered it mad it worse i have two buddy with crew cabs and they do the same i am going to try balancing or get a custom built one since i broke mine tonight on the way home

 

No bueno Heath.