1990 Ford F-150 Truck won't shut off

Hello,
I have a 1990 F150 and for the past few days it seemed like once I turned the key it hesitated a second before starting, or I had to turn the key a little bit further than usual. Today I started it fine but then it wouldn't turn off. I turn the key back and can even remove it, but the truck stays running. I bought and installed a new lock cylinder but that didn't make a difference. Help please.