If it's from an 8.5 and it's 28 spline it should work, period. I believe the 28 spline rear went up to about '88.
Keep in mind, though, the factory "S-spring" type posi units do not have as much preload on the clutches as an Eaton or similar aftermarket unit. This means the clutches are more easily disengaged, and allow each wheel to spin freely whereas many of the aftermarket units are a lot stiffer and will keep the rear axles better locked together.
Stiffer is better to a degree, but at some point you'll start getting clutch chatter (depending one preload, vehicle weight, etc).
Since the S-spring type units don't have as much preload one the clutches, and it's easier to disengage the clutches and allow the wheels to spin independantly, it's also a lot easier to burn 'em up. Many of the used units you'll find are in serious need of a rebuild.
Over time, as the springs weaken and the clutches wear, the amount of friction in those clutch packs decreases...if the posi unit has lead a hard life (i.e. iroc camaro doing a lot of burnouts) it's very likely the clutch disks are completely shot and it's pretty much going to act like an open diff.
How does that work? Obviously as wear increases, the friction decreases...now let's say the previous owner has a lead-foot and liked his burnouts. As the friction in the clutch packs decrease, the wheels are more likely to spin independently of each other--so if the previous owner does many burnouts when the posi is slipping (i.e. one tire fire) those clutch disks will be slipping at a high rate of speed, creating a lot of heat, and glazing over...
Then, VOILA! You have a posi unit that is no better than an open diff!
The reason I'm telling you this, is because those factory posi units are selling for an insane amount of money on eBay these days...I've seen several selling for $200+ in used condition. If you're lucky, it'll still be "decent" enough to just use...but there have been very few I've come across in that type of condition. More that likely, you can plan on it needing a rebuild.
A rebuild kit is $120-150 for new clutches, and a clutch shim pack--definitely get a kit with the clutch shim pack. Shim it as tight as possible *barely* being able to reassemble it. This will increase preload and make for a MUCH stiffer posi!
Figure anoth $20ish for a new S-spring, and another $40ish for new carrier bearings...now you've got nearly $400 into your "stock" posi unit--and it isn't as strong as an aftermarket unit you could have bought for $450.
Of the three used "s-spring" posi units I've dealt with, only ONE would work like it was supposed to...but it was still disapointing how easily the clutches could be over-ridden (by no means as strong as any of the aftermarket types).
One of the others was so weak that if one person held the brake drum in place, you could spin the other side freely...the other was tight enough that the tires had to be on (more leverage) before you could do the same thing--i.e. it was still VERY weak.
Again, my point is don't expect a used posi to perform as well as a new aftermarket unit...and go into the purchase assuming the factory unit needs rebuilt, as *MOST* of the time they do. If you can't buy the unit, purchase a rebuild kit, and get new bearings for a combined cost of less that about $300 don't waste your money--you'd be much happier with a stronger aftermarket unit.
FWIW, the most I paid for any of the above posi units was $75--and that was only because it came with the entire rear end. .
Just my $0.02
Just in case, this is what I mean when I refer to an "S spring" type posi unit.