2001 Chevy Astro Rear end noist

I've had rear end noise - very noisy for 2 years. The mechanic says the rear bearings are worn out and the gears and making the noise. They want to put in an 85,000 mile junk yard rear end for $1100. I don't think it's worth putting that much $$ into the van.
When the rear end finally dies out - how will it happen - lock up at 70 mph, just run free?