70's era 12 bolt Posi tough to find?

[siggyfreud]

Hey all,

Searching for a rear for my truck. Trying to convert from 8 lug to 5 lug. Went to the Portland swap meet, walked around for nearly 4 hours and didn't find a potential rear.

I found a few on Craigslist locally:

http://portland.craigslist.org/wsc/pts/4367497095.html
http://salem.craigslist.org/pts/4392592465.html

However they are both open diffs. Are Posi's difficult to find stock? Worth buying one of these and replacing the carrier and ring/pinion with the gearset I want, or should I hold out and find something with a posi unit already? Not sure how rare they are, so hard to know which path to take.

Thanks!