Re: posi-trac, lockin' diffz...
I know you want to go with a used unit, but frankly I'd get a new one. You don't know how a used unit has been used (or abused), and this way you can order it with the ratio you want along with the proper ring gear. A good site to check out is Yukon Gear & Axle
even if you don't want to purchase, it can still tell you what is available for your vehicle and other vehicles; makes a good reference site.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!