Talstaff

I have recently bought and fited a second hand diff to may car, the current diff has been on its way out for a long time, when i removed it, it had a horrendous ammount of play either side and had been leaking oil. The new diff 20000 miles went on with no problems at all, however when taking the car on a test drive it was very smooth and quiet compared with my old diff but when i get to 45 - 65 miles an hour there is a very noticeable vibration the most obvious cause to me is the propshaft balance or alignment there is 3 bananna shaped washers on the end of the propshaft where it bolts to the diff and a weight welded to the propshaft itself, the new diff i am sure is fine, and the problem i am convinced lies in the propshaft. has anyone had any experience with this my initial thoughts are to rotate the prop through its 6 possible positions any advice would be greatly appreciated.

Chris