Originally Posted by nuadu78
I mean, I know what I need to lower that thing
Lowering a t-case is to fix a specific angle problem. You do not know if you will have the problem or not.
When a smooth output of a t-case goes through a u-joint bend, the momentary speed of the drive shaft (degrees of rotation/time) varies, meaning the drive shaft goes slightly faster and slower during each part of a revolution. A correspondingly equal bend as the drive shaft attaches to the axle pinion perfectly compensates and the pinion spins as smoothly as the t-case shaft. If the bends are not equal, that speed difference shows up as a vibration.
After a lift, the drive shaft bend at the t-case and at the axle should be exactly the same, meaning the output shaft of the t-case and the input of the axle are parallel (not on the same line, just parallel), it should be fine with a 3" lift. If the lift gets tool tall (the parallel lines get further apart), the bend angles of the drive shaft, although still providing a smooth ride, start to cause a short life spans on the u-joints.
Lowering the t-case (tipping it down), and a corresponding altering of the axle pinion angle (tipping it up) to keep the lines parallel, simply makes the two parallel lines close together so the u-joints last longer. In most cases, 'vibes' are due to mismatched angles. A t-case kit can reduce the mismatch and eliminate a good part of the vibe, but at the cost of reducing clearance. Altering the pinion angle is a better way to fix the vibes as it keeps good clearance.
Do your lift. Keep the lines parallel, and you should have no problem with only 3 inches.