Reason for car not shifting any gears? please answer?

Okay so my rear end is bad on my chevy suburban. Its been like this for 5 days, and I was still driving with no problem. It was making a very loud clanking noise coming from the back. My u joints are good. 3 different mechanics all told me that I needed a rear end. Now my truck is not moving, no matter what gear I put it in. When its in park, there is a very loud noise coming from the drive shaft. I know I need a new rear end. But why is the vehicle not shifting gears now? I mean it shifts but just won't go anywhere. Why?