My 84 also has this irritating problem(besides the thump a lump) of not turning off when I want it to do so. I can turn the key and the engine cranks once, maybe twice and fires right up. When I turn the key off, it doesn't shut off the engine. The electrical stuff like the gas and temp gauge will quit indicating when the key is turned off, but the engine just keeps running. I have to stall it or pull the coil plug to turn off the truck.
The previous owner of the truck told me it was the ignition contacts. The fix was to purchase a new starter. I did this, and put the starter in today (god what a pain, and I've heard the 3.0 is worse!). Well I wasted my money and time apparently, because that didn't fix anything. I also replaced the wiring harness inside of the steering column. Its the thick pack of wires with a harness on one end and a circular piece that connects to the ignition key's mechanism. This didn't solve my problem either.
so, if my rant/problem makes sense.. Does anyone have any experience with electrical problems like this? Any suggestions?