SOunds like something simple like a bit of pocket fuzz jambing a tumbler. Try some compressed air first. If it doesn't free try a squirt of brake cleaner or electronics cleaner. No to WD-40. Insert and wiggle the key a bit to get things freed up. Once completely dried and free use a spot of graphite powder. The keyways rarely fail without hints but they can be fully disassembled if necessary.
You can remove the steering wheel cover to access the keyway
It wraps around the collum
And is held together with two bolts
They can be awkward to remove but no big deal
Then take it a lock smith
Or get a replacement
I broke the key off in mine once and tore it up to start it
Mine was a 91 also
But it has a plug at the key
You can unplug it to start it
And in the key collar there is a pin that locks the steering
Same thing happened to me on '93 earlier this year. I was stuck, and while I know its not recommended to do this, I had no other choice, save for an expensive tow. I shot it with silicone lubricant by inserting the small red attachment that comes with these cans into the key tumbler. I also removed the black rubber surround on the tumbler, and really lubricated the entire thing. It worked. When I got home, I ordered a new tumbler from a Rover junkyard, and it still sits in the back of the truck. I have yet to have another issue with the ignition. YMMV- and I never like to tell others what to do with their vehicles, just sharing what I did, and what worked for me.