You could, but would could you possibly gain by doing that?
Unless you want the Solid Axle off the '85. In which case quite frankly I would just swap the Axle onto the '89.
The '89 had larger brakes (a popular upgrade on '85's and '84's), stronger power steering (again an useful upgrade), and the Rear Axle is slightly wider and a bit more gusseted than the '85. Many hardcore rigs use rear Axles off IFS trucks/4Runners.
To be frank, you'll end up swapping a lot of more off the '89 than you would if you just put the Front Axle on the '89. That is if you want to do it right. If you have no intention of ever wheeling it, then I guess you could do it. But it sounds like a lot of work for very little payoff.
Either way good luck!
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