Hi /auto/

Those things climb mountains in 3rd which is why I believe OP's truck has some issues.

If you put bigger tires on change your differential ratio. You should be able to get parts and work done at a decent place for 2k or if you have patience and time parts can be had for probably 1000

They don't climb mountains at 50 to 60mph in the high range box user

If you haven't done so, give it some good maintenance/tune up. It's almost a 30yr old engine.
>Get some upper cylinder restorer/soak and get those cylinders, pistons, and rings free of all that carbon.
>Pull the injectors and send them to get cleaned or buy new ones. It's been 30 years so maybe new injectors will atomize better?
>Use a engine flush a couple times with some cheap oil. This should cleanup any gunk in the engine. After you flushed used your preferred oil. Same with the tranny and diff.
>Get a CAI. The stock filter system on those old beaters are all tuned for maximum efficiency/not power. Thanks DOT/EPA.
>Check your wheel/axle/driveshaft bearings.
>Change/check sparkplugs, boots, coils, distributor.
>Check your belts, chains, and the accessories like power steering pump and AC.

And once you get that done you can work on cheap mods.

Simple things like a cat back exhaust and intake should give you some gains especially on a econobox truck.

Though I'm not familiar with any of those engines but I'm sure you should be able to find compatible aftermarket parts like headers and intake manifolds for dirt cheap. Hell maybe even the newer re engines share the similar components so a simple used head swap might give you good gains.

Join a forum for your truck. Alot of info is probably already documented plus they'll help you out too