Hey I just put 33's on my 1990 Full SIze Blazer and a 4 inch lift. I also changed the driveshaft cause mine was missing a piece out of it. Also just after I put the lift and tires I got into an accident and bent the front driver side corner frame. I got the frame bent back to normal so it still needs a front end alignment from the accident and with the new tires on it. But ya thats the details and so now today I got it goin again and drove it and when I get goin there is a vibration at low speeds but once I get goin up around 30+ it smoothes out. So my question is what could possibly be causing a virbration at low speeds. You can feel it throughout the truck (floor, pedals, seat, not so much in the steering wheel.) But ya any suggestions on what it might be?

broken tranny mount
unbalanced tire
bad u-joint
too much pinion angle
if you have not dropped your tc case that will help also.