You've just gotta talk to the person selling it long enough to figure out if it is a good deal or not. The guy I bought mine off of put 4.10's in it, and broke them. GM set it up incorrectly when he got the gears replaced, so they gave him a brand new rear with 3.42 gears. That is the one he sold me.

You just gotta know what you're buying. The junkyard idea would be even better. At least with a stock rear you know your gears aren't going to howl. Having a shop install new gears is kind of a crap shoot as far as gear whine goes. Plus you've got to spend the time breaking the gears in...

Replacing the axle is not as hard as you think. Unbolt suspension, unbolt driveshaft, take off brake lines, unbolt a few other minor odds and ends, then the rear just drops out. Put the new one on the jack, and do everything in reverse.

The hardest part of the whole thing is putting the e-brake on, and to be honest it took all of 10 minutes. We were "first timers" and did it in 4.5 hours. Changing spark plugs is harder than swapping 10-bolts.