How long do I really have? (Rear Differential Bushing)
 

I have a 2006 SRX 3.6L V6 with just over 90,000 miles and I am the second owner.

The Cadillac dealership says my rear differential bushing needs to be replaced. They wouldn't get into details, only to tell me that it is an important repair that needs to be done quickly. I know they're pros, but I also know they're salesmen, and at $600, I just want to know how serious the problem is and how quickly I should get it repaired. In the past week, I've noticed a thump coming from the rear of the car when I take it out of park and put it in reverse or drive. I have also noticed this thump happening when I press on or let off the accelerator. So, my questions are:

1) Is the thumping noise consistent with the rear differential bushing needing to be replaced?
2) Under normal driving conditions, how long do I really have before more damage is done to the car?
and 3) Pay $500 at the dealership or $300 at a private mechanic?

Thanks for any input.