I figured I'd put this in the GM section because the Ford section doesn't get alot of traffic and the theory is basically the same for either vehicle.

My mom's 2006 Ford Supercrew has some vibration at about 30 after I put some 2" drop shackles on it. Do I need axle shims or pinion shims?

 

Axle shims to correct the pinion angle...... pinion shims are to adjust pinion depth inside the differential.