regular rear dif to posi-trac

[KyFarm]

Ok, I have a 72 c-10 4x4 lwb that has a crap body and a real loose motor. It has a good TH350, NP205, tilt steering column, and good axels and brakes. It was advertised as having 3.08 gears and has 32" tires.

My '69 K-10 swb stepside has a good body, good power steering and a decent (for now) motor. It however has the 3 speed on the column and 4 drums with no power assist. It was advertised as having 4.11 gears and has 32" tires.

My plans are very soon to put the TH350/NP205, steering column and the front axel from the 72 with all the power brake parts in the '69. Then I need to either also swap the rear axels to make the gears match OR

Here are my questions finally. Is a posi-trac rear a completely different axel or just the gears in the rear diff. And if just the gears about how much are a set and can they be bought new or do i have to find a used set?

Both trucks have the standard 12 bolt rear axels. Thanks for any help