Carrier bearing

Hello, my car started this intense vibration/shudder under slight acceleration awhile ago, I took it to a mechanic. He diagnosed the problem as a failed carrier bearing and worn u-joints. I said go ahead and had them replaced and the car drove like a dream for the first day but now the shudder has returned, it's really felt in the center console area and center of the dashboard. It can be felt in the front seats but not the steering wheel. It increases when accelerating up an incline but after a few moments and once the car gets above a certain speed it will just slowly fade away. Or if floor it, it doesn't happen what so ever. It doesn't happen all the time, it does it at random times. I'm tired of playing the guessing game, this is getting to expensive. Do you have any idea whats going on?