Used gears ... in my book are a pain to setup (the ideal is that the original wear-patterns and your new setup wear-pattern should match 100% to avoid hot-spots related to the wear and work-hardening of the gear faces).
The 28-spline bit relates to the axle shafts used, the interior spline count on the diff's side-gears would need to be compatible with the splines on the axle shafts that would fit into those side gears -- the shafts on rangers are 28-spline (so you should be good if you have a Ranger 8.8.) -- the Explorer rolls with 31-spline axles, FWIW
As far as my opinion
on the value of this Craig's list ad...
... I give it a 'prolly-no-thanx' rating based on the following:
- Used gears; I'm not a fan, worth a lot less than half-new price (value at $50 max)
- Used LSD; Value should be about half of new (you don't know how this was treated, the clutches could be noisy or excessively worn) - a complete new carrier can be had in the $240 range or less if you shop
- The hassle-factor of "not knowing" the bearing condition would indicate that I'd need to pull them to replace them (new carrier & pinion wouldn't need to have bearings split/pressed off'a them...)
From where I sit (hopefully, not sounding all 'high-&-mighty', really not my intention here) I'd say that used Gears on Carrier would be worth a maximum of $150 IF
the seller has a clear paint pattern on the gear-set (to help you match on install) and that before you get to looking at hassles and the unknowns