If you have a 2 piece drive shaft and you've lowered your truck you may experience a low speed (0-20mph) flutter or slight vibration during take off. This is almost always caused by the "Carrier Bearing" (where the 2 drives shafts mate) and can be easily fixed with the shim kit. What your doing is spacing the factory carrier bearing up with the shims, towards the bottom of your truck. This recreates the original drive shaft angles through the bearing and the vibration goes away.

The vibration doesn't always happen, sometimes with a 6" flip kit, and even with a 4" hanger/shackle kit on very rare occasions.