That sounds like the switch isn't turning off occasionally. Not at all uncommon. If you ever disassembled one, you'd see why. Have a friend, wife, somebody start the truck and then turn it off. When it continues to run, disconnect the wire leading to the coil...either one of them. There'll be + coming from the ignition switch and - heading off to the distributor and when either of those is disconnected, the engine will stop running. Personally I'd disconnect the + wire. If the engine dies when you do that, then disconnect your battery and check your ignition switch with a handheld tester (one with a broken circuit checker). Test between the center wire (the hot wire) and the wire going off to the coil. You'll see it turn on (or hear the little buzzer in the tester). That shows a completed circuit. When you turn the key off, you will still hear the little buzzer...maybe. Let us know.
