vibration problem

I have a 76 Duster I bought 10 years ago and it had a 225/904 combo with a junk rearend. I built a 440 and put a 727 tranny in it and had Moser Engineering fix up a 8.75 rearend for it. I put a set of 3:55 gears andit all seemed ok. I decided to restore it and every nut and bolt came out of the car, dipped repainted the whole ball of wax. Put the same engine back in, same tranny and same rearend. I decided to go with a 4:10 gear. got the car back together,painted , and now I have a vibration that seems to be in the driveline. I thought it may be u-joints so I took the driveshaft to the driveline place and told them to replace the u-joints and balance the shaft. He told me the driveshaft was too short by looking at where the boot rubbed on the shaft. Measured it and lengthened it 3 inches. Put it back in and still a vibration. Thought something may be in the rears so I switched rears and put a 3:91 punkin in it. Still vibrates. Could something have happened in the tailshaft by running a too short driveshaft or in the tranny itself. The tranny runs perfect and shifts so smooth. Am I missing something. About to pull what little bit of hair I have left out. Any help will be appreciated. Maybe someone else has had this. Bill