97 Tahoe, 4x4, with bad axle?

I brought my truck in to a shop because the rear axle is making a weird grinding noise. The noise is only there when I press on the gas and, it's mostly there above 20 mph.

Anyway, the guy said that it sounded like the bearing in the axle are bad and that it would be easiest to get a "new/old" on from a junk yard and have that intalled.

How big of a job is replacing the rear axle? In other words, how much am I looking to spend on this?

thanks,
dj