Vibration Problem

[autobodyman2]

for some background info, I have an 82 chevy k 30 dump. It has the 14 bolt rear. I had to replace the tranny and ever since then, i have a bad vibration from about 25-45 and quits after that. It will also stop if I push the clutch in. I believe it to be coming from the rear end. How true is it that the driveshaft has to be marked before you pull it back out? I have had it out and in a bunch of times, and maybe i screwed up there, any help is appreciated.