1991 f-150 Starting Issue HELP!!

I am having a major issue with my 91 F-150. After driving all day, I started the truck to go out this evening. It was revving unusually high, and when I turned the key to the off position, it didn't turn off. Instead, it kept trying to crank over. I ended up having to carefully disconnect the battery just so it would turn off. Has anybody ever have this problem or know what the cause is. Believe it or not, I've only had this truck for 36 hours!!! Any help would be greatly appreciated.

Ignition switch or solenoid?

I had a similar problem on my 1990 F250. It just died on me one day and when I tried to start it it kept cranking even with the keys out. It ended up burning out my selinoid and electronic ignition control modual as well as my computer. When it was hooked up to a computer it originally told me my ignition modual was shot. after replacing this it still would not start, we then traced a line back to the computer, replaced it then it started like a charm

Also make sure your battery is at full charge.
A low voltage battery will cause the starter solenoid to stick on for some reason.
MT

I replaced the solenoid and that seemed to do the trick. Thanks