i need to know how to tell the difference if i am buying a posi or non posi unit?any help would be great.these are used units.

 

I think all Fords use a metal tag that tells the ratio and whether or not it is a limited slip. In the top right corner the ratio will be expressed as (example) 3.73 for an open diff or 3L73 for limited slip. Hope this helps!

 

Have someone hold the pinion shaft. Turn 1 wheel. If the other turns in the opposite direction, it's an open rear. If nothing turns because the pinion shaft is held it's a Trac-Lok ("posi" is GM-speak, please don't use that kind of language on this board) to confirm turn the pinion shaft while holding 1 wheel. If the other turns it's an open. If they both want to turn in the same direction it's a Trac-Lok

 

the last advice is sound. Just remember that "posi" is overated; these units use clutches that wear out and used units typically don't have allot of life left anyways. Its usually a better bet to spend less coin and pick up an open unit then throw in a spool, locker, or whatever. JMO.