Axles must come out so the diff cover must come off and the c clips removed,
check with your aftermarket supplier, they make 2 piece backing plates to
make this simpler, made by Dorman.

I recently did the same thing. I have a 2002 Z71 Suburban. This has a
locking rear differential. While you are pulling the axels replace the
seals. The whole job took about 2 hours, including replacing the rotors
with new OEM (Better quality than any aftermarket) OEM pads, and
replacing the caliper locating/sliding pins and seals. I'm not sure
about the "positraction" rear you are talking about. The locking rear
diff. is very easy to remove the C clips. My advice is to go to your
local chevy parts counter and ask for an exploded view of the rear. They
have these diagrams available for free. You just need to ask.

Remove the center pin of the spider gear housing by unscrewing the
retaining pin that goes through it. Do not rotate the spider gear
housing once the center pin is out. Push the axle in slightly to expose
the C clip. Sometimes the axle needs to be guided into the opening in
the spider gear housing while pushing. Better to have two people to do
this. The shaft seal wants to push the shaft back out, so it will not
stay by itself. I had to use a magnet to pull the clip out, but again it
was easy. If you have a true positraction, I would expect to find clutch
packs that need to be compressed to get the axles out. I was lucky not
to have these. One of the mechanics at Sun Chevrolet, Bohemia, NY told
me about this, so I have no direct exp. with these. Locking rear diff.
has a govenor that spins, and when it spins fast enough (one wheel
spinning faster that the other) it solidly locks the axles together.

Hope this helps.