It could be from the R&R of the exhaust. Check the exhaust isn't touching anything and that it's still nice and loose on the rubbers.
Also check that the flange at the front of the gearbox it TIGHT (the one connected to the rear donut). If this item was removed, it may have worked loose. If it lets go there will be much unhappiness
Could also be from a binding locator in the propshaft. It's been ryears since I had one apart, but there is a spring loaded spigot which I think is inside the front end of the shaft. The thing should slide freely but if it kicked off on installation it might cause a vibe.