Oh, I missed the '89.
Okay, here's the deal. The older axle should bolt right in as-is. The brakes are 10x1.75" vs 9x2.5". The axle should be a non-C-Clip axle. There are retaining plates for the axle shafts. I'm pretty sure you can mix and match brakes, but I'm I've never done it and haven't heard of anyone doing it. If there is a difference it's going to be the actual thickness of the backing plates which I suspect are the same though. Other differences is with the carrier and side gears. Your old ones are ABS version while the '89's are non-ABS versions.
Kinda strange that you'd find an old '89 and nothing newer.