3.0l Vulcan engine is a reliable engine, your '91 was the first Ranger year that came with 3.0l, it replaced the 2.9l, it has a distributor, in '95 Ford changed over this engine to distributorless.
Ford used the 3.0l Vulcan engine in cars, vans and trucks since 1986, so there are a whole lot of them out there, but there are not alot of upgrades for these engines.
Just keeping them running correctly is about the best you can do.
General power upgrade for Rangers is to install an Explorer V8(5.0l) engine/trans, wiring and computer, explorer and ranger had similar dimensions, but still a tight squeeze.
'91 Ranger 3.0l would have A4LD 4-speed automatic, it is a good transmission but like any and all good parts they do wear out, so people complain.
"Hey my 20 year old transmission broke, what kind of POS is this", lol.
Good read here on the A4LD: Ford Ranger A4LD Automatic Transmission
Switching to a different model trans would be..............well harder than just putting in a different transmission, automatics are computer controlled since the mid-80's, same computer that runs the engine, so you have to change that, and then alot of the engine controls and the speed controls to what the new computer needs.
You have a TFI ignition system and at 24 years old it will have had some problems, parts have probably been replaced already, but as time marches on...........
Good read here on the TFI system: Ford EEC-IV/TFI-IV Electronic Engine Control Troubleshooting
This is for the head liner: Re-covering your Ford Ranger headliner
They are just called Armrests, and you can buy aftermarket replacements for most models.
Is the rear window lock the slider window, or is the the pop open rear window on a extended cab?
Either should be replaceable without removing window.