I have an 86 Z28 with a 305 right now, and the rear end (2.73) is on its way out. I don't want to spend too much money on swapping it, so I've been looking on craigslist for a cheap replacement. Would it be possible to swap the diff for one from a 91 with a V6 (3.23)? What would it require to swap other than simply removing it and putting the other one in? Thanks for you help!

You are talking about changing the complete rear end, right? Just swap it out. Nothing to it. The '91 will have 28 spline axles, so that is an upgrade from your 26 spline axles.

If you have drum brakes and the '91 has disc brakes then you will get into more work chaning it over.

Sounds like he's talking about the carrier, ring and pinion. If so it's a fair amount of work to do it right. Read lots on this board about how to set up the gears etc. I'm going through it myself for the first time. I had to rebuild the posi unit too. All interesting but slow going first time.
And that's post 1,000!

No, it is a direct swap if you use the same drum brake style rear. Bolts right in and looks identical.

I meant replacing the entire thing. But that's great that it shouldn't require much. How high should I get the car up and should I get another person to help me out with it?

There are many posts, pics and vids on how to remove and install rears on these cars...Just do a search...

That's what i meant too. A v6 and a v8 Fbody are the same car. Same rear. Entire thing bolts right in. Drum to drum

If its stock than your still getting a rear without a posi. The higher gear ratio means its easier to get a one wheel peal. I have heard here might be a locker you can put in with out tearing it apart but someone else will have to chime in to answer that and give details.

Power Trax Lock Right goes in the standard differential case and works like a Detroit Locker. It's pretty easy to install, but can pop and jerk some. Not very noticable with an automatic, but more noticable with a manual transmission.