How big of a job is differential bearings?

[jbach36]

I just had my transmission rebuilt (blah). But I still get banging on shifts especially from 3rd to 4th gear. The mechanic said that's not the transmission but probably the differential bearings. Why would they bang when the transmission shifts? If this were a vacuum issue, it'd be in every gear, but it's pretty much just 3rd to 4th.

1991 300d, 258k