do you want to swap gears from one third member to the other third member (or pumpkin as it is sometimes called) or are you just going to swap the third memebers? easier to swap the third memebers as long as they are both from the same style of rear end and use the same spline axles (should be 28 if stock 9" rear end).
swapping gears is a little more complicated and you will need a dial indicator, marking compound and inch pound torque wrench to set them back up properly. lots of articles on the web on how to do this, but it does take some time and possibly some new shims to do it properly.
if just swapping third members, all you will need to do is unbolt, clean up mating surface, new silicone, install.