New dif shudder/vibration?

[Racer-X]

New front dif installed late last week. I'm feeling a little vibration (best i can descibe it) 20-30 feet after taking off from a stop. Then it goes away when you get up to speed. It also does it when you come to a stop in the same speed range.

So it's obvious something has been done wrong but any ideas what? I want to sound edumucated when I take it back in.

More info: They changed both sides of the case due to the crack, but used the old insides.

Not 100% positive its not the new tires (BFG MT) I'm feeling since both the tires were put on and the front dif changed within 20 miles. But since it's only a very low speed that I feel anything I doubt it.