Been looking into this....
None of these units tested use clutches...
Posi Unit Shoot Out
Though the Alabamaman setup has hardened side gears that come with.
So basically all those units are a fancy spring.
The Ford "S-spring" "preload spring" "wave spring" or whatever you want to call it from their Traction-Lok diffs is a less fancy spring, with a less fancy price. Apparently the one for 7.5 inch Ranger L/S diffs is the strongest stock one that is meant to "drop in" to our diffs. The 8.8" ones apparently need some grinding to fit, but come stronger for F-Series HD applications. Ford Racing may have a choice of spring tensions for these.
Anyhoo. 7.5 Ranger traction-lok s-spring is meant to be under $20 and available at your phriendly phord dealer.
Coles notes application to our diffs: Pull diff, pull diff pin, hammer in s-spring, replace diff pin, replace diff. Side gears may need a ridge grinding off the top of them YMMV.
So this be what I'm about to inflict on my A670.
Edit: just found out it's also called a "differential clutch spring" per the FSMs for 'stang, tbird, crown vic....