I have a slight vibration with the 3 inch blocks in the rear. The vibration is from about 45 to 55 mph...is this common???

Mine did the same thing with 2" blocks. I ended up lowering the carrier bearing 1".

You might have a bad angle for your driveshaft. Did the blocks you installed have a taper to them? If so, did you mount the thinner side facing forward or backwards? If the block didn't have a taper then you might want to swap them for a set that does, because one of the first signs of a bad driveline angle is a vibration.

With a 2-piece drive shaft, you need to lower that carrier bearing to get the right pinion angle. Still may need to shim the rear axle aswell.