The only thing that can keep you from rebuilding that t-case is the cost of any tools that you may buy, the rebuild kit and your desire to learn something new.
Go for it!
On another note, the flanges on that '87 will not match the rest of the '91 truck (at least the rear driveshaft in your case) because the flange size changed during the great '88.5 model year or thereabouts. That's not the end of the world, though. You can find flanges in the junkyard that will probably work fine. It would be helpful to find an '87 intermediate shaft to cut down on the flange replacement situation.
Good luck!