I have a 1981 GMC 1/2 ton with 305 engine. I use the truck on the ranch as a work truck. About a week ago when I tried to crank it the starter caught and the truck started but the starter kept spinning. I turned off the key but the starter kept spinning. I immediately (let's say as fast as I could) disconnected the battery. I reconnected later and all seemed okay. Then it did it again and I disconnected again. Should I a: replace the entire starter and solenoid
b: only replace the solenoid

Thanks

is the key sticking in the start position. sounds more like ignition switch.

No the key is not sticking. I can turn the key off, the engine stops and the starter just kept on spinning.

solenoid on the starter.