Are the buttons in good shape? Every old Ranger I've ever seen, the buttons were tore to heck (sometimes still work though). You can get new ones cheap and they are pretty easy to swap out, so you could eliminate that easily.
That sure is low miles though, assuming that's the original odometer.
I have shop manuals but probably no help because, I think, your '91 is a different gen. Don't know how different it is from mine. If you could get ahold of the shop manuals they usually have a diagnostic routine to find where the problem exists.
Off topic, but just reminded me of fixing cruise on an '86 Saab manual trans where they had a ball chain arrangement whereby the cruise locked onto the right place on the chain... it was weird, but it worked.
New trucks have + and - for 1-mph incremental up/down which to me is nuts because the '97 already had that, you use the already existing buttons to creep the speed up or down, it works slick as can be.
I guess now it's too hard for people to read the manual.
Do your buttons light up? Some years do and some don't.
If the cruise light comes on the dash but cruise itself doesn't work that would tell you something probably; I assume you get no dash light when you set speed. Assuming it's like mine you have to turn it on first then set speed but I'm sure you know that. Good luck I'm sure you can fix it.