I still say this is a bad move, with the weight of a GC, once you put in a lunch box locker it is only a matter of time before.....
Having said that, jack up the vehicle as high as you can and support it with jack stand on the frame, pull the wheels and allow the axle to droop down as far as it will go. this gives you the most room to work in the differential. the PowerTrax is not that hard to install, read the directons twice before you begin. You are really just changing out the spider gears for the locker gears, you pull the center pin and the spiders will just about fall out, then the C-clips and then the side gears and the axle just slides out. while you are in there you should change out the axle seals as they are cheap and you need to pull the axles anyway. this should not take you more that a half day, just take your time and follow the directions.
Once you are done, remember to go easy with the skinny pedal other wise all that work will disappear with a loud BANG!