If you're doing ring and gears, Hills Garage might be your safest bet even though it is a little bit of a trek being in MD.
But just swapping the diff, then you should be able to find someone local that can recommend a competent shop. Consider posting in the regional forum (mid-atlantic? nova?). Or have you tried searching the forum yet? There's a lot of Z owners in your area; I can't imagine the question hasn't been asked before
