i feel a slight vibration when i acclerate at certian speeds, my truck is lowered so im wondering if i need a shim, and how much of one i would need. .

 

No way to be sure unless you measure the angles of your u-joints. You want them both to be about the same, unless you have a double-cardan type driveshaft. In that case, you want the pinion about one degree down from pointing straight at the transmission output.