Will an 8.5 28 spline posi with 3.08 gears from an 86 C-10 truck fit a 71 Malibu 8.5 28 spline 2.73 open rear?

it should

I think there's a difference between truck and passenger diffs. I'm not sure exactly what the difference is but when I was shopping around for a locker I saw two part #s for truck and car.

I thought the truck axle was a 30 spline?
Mike

12 bolts are different from trucks to cars, but an 8.5 is an 8.5. they were all 28 spline axles up until the early 90's, then they all went to 30 spline. i hear that even differentials and gears from a newer truck 8.6 will work in an 8.5 housing as long as you have the proper 30 spline axles.
but i don't believe there is a c clip style 8.5 housing for a 71 Chevelle- and with the exception of the cover and axles, 8.5 parts will not fit an 8.2 housing so it's all kind of moot.

Yes, the parts from the truck 8.5 will fit the Chevelle 8.5 rear end. I don't recommend using the truck Governor Lock differential in a performance car because there are several small parts in it that will break easily, but you can run it until it breaks. You are probably going to find that the '71 rear end doesn't have c clips. Don't worry, the parts will still work. Just use the Chevelle axles the way that they are. If there are no c clips in it then don't try to put any in it.