Drive line vibrations

I just swapped rear gears in my car. 73 duster 360, 727 trans I took out a 741 case that were 3.23s and i put in a 489 case 3.73s. A rebuilt case with brand new ring and pinion and all new bearings.

When I went to put the drive shaft back in I realized the u-joint was to small for the 489 case and I found one from moog part number ( moog 347) that has the 2 different size ends combination u-joint I think it's called and it fit perfectly on the case.

Took the car for a curse around my neighborhood to start breaking in the gears my neighborhood loops around a lake so at 25-30mph the gears sounded fine no noise or vibrations let the gears cool down then I took it out on the main road. Everything sounded fine untill I was getting up to about 50mph then i felt a pretty strong vibration. Only happens when I get up around 50mph.

I have not checked my pinion or drive shaft angles yet i have a list of things to go over and check. But I'm wondering since I put in that u joint to fit the 489 case if it threw off the balance of the drive shaft. Any and all help, information and input is greatly appreciated.