I was test driving my A6 C5 2.5 TDI Quattro on a dual carriageway yesterday at 70 mph (ish), I put the hammer down, the gearbox kicked down and there was a sudden loud bang from the rear of the car which felt like it came from the rear diff, I let the power off and drove very gently at much lower speeds until I got closer to home and then give it a bit of welly again but it didn't do it again.

It does have a bit of weep from the right hand diff seal but not dripping, could it be low oil? could this be the onset of terminal damage?

What will happen if the diff goes completely? wheel lock-up?

It's an Auto Tiptronic box.

Concerned....