Driveshaft length
I JUST got through resolving the same problem. I put 4" lift on, springs up front, blocks in rear. The guy I bought it from had no problems with length or pinion angle. No one else ever did ever, but I did. I had a huge problem. My driveshaft stuck out far too much, but did spline up, but when I went to stop there was a huge vibration and horrible noise. It felt and sounded just like a brake problem, but it turned out to be a driveline problem. The driveshaft wasn't in far enough and caused a binding under weight transfer during breaking. I did a transfer case drop, and it helped, but didn't fix it completely. I had to have my blocks machined to change the angle and everything is perfect now. I have about 1/4 to 1/2 of driveshaft further out than before.
We believe the reasons for this is that my rear springs had sagged severely, causing sag and pinion angle problems, thus length problems. The measurements that worked for me were 1" transfer case drop and then the measurements for my blocks were 3 inches in the front, 4 in the rear. I experimented with temporary wooden blocks to get the right angle.