Level of difficulty changing 8 3/4 gearing

You might try raising both rear wheels with the trans in neutral and spinning a tire 1 revolution and count how many times the drive shaft rotates then say it's 3 1/4 turns it 's 3.23s etc.

I believe it's easier to change the whole center section (pumpkin, hog head). You can usualy pick them up reasonabl and just install it in your assembly. Basically you carefully slide the axle shafts out (enough to remove the centersection) then put the new one in, replace the shafts carefully (so you dont tear the seals. You will have to set endplay on 1 side.

Personally I would have a machine shop set up just a gear change if that's the route you go( special tools & equipment,as mentioned dial indicator, press etc).. You could remove the center section and bring it to them then to install the gears then you re-install the center section.