Rear end on way out or tranny?

Got a 96 a4 trans am that I've been driving for the past 40000km s with a noise coming from the back right rear tire, at first thought it was the bearings in the rear so I had those replaced and didn't fix the issue, next thought it was the back wheel bearings so took it in to a transmission shop and they said they were fine but thought my crown and pining gears were worn out now after driving it like that for so long my transmissibon is a slipping on 2-3the shift and barely goes anywhere in 3rd or 4th, tranny is only a year old with with a cooler deep pan and the fluid is bright pink so is it possible for ttranny to act like this with a bad rear as well? Can a 3rd Gen body rear also fit into a 4th Gen car ? Also forgot the grinding noise has always been occurring from the rear only in 4 th gear and reverse now is happening a little in first as well!