8 1/4 rearend posi unit

The only thing specific about the Dakota diff is the axle-shaft spline count as posted above. A Sure Grip from a '73 Duster or '81 Diplomat will work in an '87-'96 Dakota, but not newer.

The only other difference to consider is that there are two different types of Sure-Grips: the Borg-Warner (now Auburn) cone-type unit, and the Dana/Spicer Trac-Lok (clutch type).

The Borg-Warner design is virtually invisible in use, meaning you never feel it engage or disengage. It's also fairly bulletproof if you maintain it, which just means lube changes at recommended intervals. The downside is that it's non-rebuildable (per its designers) although some have gotten away with milling the backsides of the cones to restore function. Doing so increases the clearances on the side/spider gear engagement, though, so it's still not ideal. That being said, I've had untouched B-W units with 150,000+ miles on them that still worked perfectly.

The Trac-Lok is easily rebuildable when it wears out, since the clutches are multi-plate units that are readily available and can be replaced. However, the Trac-Lok is not as seamless as the B-W unit in its operation. Engagement/disengagement is more noticeable, but it also locks harder. Trac-Loks in general--not just the 8¼" one--are also semi-notorious for clutch breakage, particularly in hard usage. Broken clutches won't stop you dead, but you won't love the noises from the rearend.

I believe all the factory Sure Grip units built for 29-spline axles were Trac-Loks, but I'm not sure of it. Trucks ain't my thing.