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First off, I would check the fluid level. Smell the fluid. Does it smell burnt? If your fluid level is good, and it's slipping, that probably means your tranny is on it's way out. I nursed a weak tranny in my old 86 F150 for 10,000 miles just by using stuff called "Trans-X". You can find it in your local auto pats store, it's in a blue and silver metal can. I would dump a can of that in, and my tranny would shift fine and not slip for 2 months. When it started slipping again, it got another can of trans-X. Of course the tranny needed rebuilt, but budget wouldn't allow that. Just throwin' it out there............
Sometimes a fluid change will help the tranny shift better. With your year tho, chances are your tranny has a lot of miles and is ready to go.
If you drop the pan to change fluid you can inspect for particles etc that will tell what condition your tranny is in. Worst case is you discover that your tranny does need a rebuild. Then you need to decide whether to ante up the $$ for a rebuild, about $2000 more or less, or just button it up, put in new fluid and hope for the best.
Best case is that you just need new fluid.
Normally I don't like additives since in my opinion, they are a bandaid temporary measure. But, if your tranny is this old, you might use an additive to help it limp along for awhile.
If it was me I would drop the pan and change fluid. Then if I couldn't afford a rebuild , if necessary, I would button it up and maybe try an additive.