Hi.....
Had this problem recently with my 90......
You can check the uj's By holding onto the propshaft and giving them a good wiggle, you should be able to tell if there is any slack in there....
The other thing to check is that the yokes on the propshaft itself are inline, as if not it will cause vibration,
realligning the yokes on my rear prop solved the problem for me....
Hope this helps
Charlie