1995 K3500 6.5 Electrical Issue

[ibanezer]

I recently bought this '95 K3500 6.5 diesel truck.

http://louisville.craigslist.org/cto/3398593302.html

The problem with it not starting ended up being that the starter was bad, which I replaced and had it running great. I drove the truck for 3 days and ran into a weird problem that no one has really been able to give me a good answer to.

I drove the truck to work the other day, 30 or so miles round trip on the highway with no issues what so ever. Shut it off and tried using a spare key that came with the truck to restart it. It wouldn't even turn in the ignition.

Here's the issue: When I put the correct key back in, it turns in the ignition, and I get electrical power(headlights, raidio etc.) but no dash lights and the truck won't attempt to start. The key may just be a coincidence, but I was thinking maybe a messed with some type of security system so I unhooked the batteries and still nothing. I'm able to arc the starter, but when the motor starts to run it basically just shuts itself off.

Any ideas as to what this could be? Sorry for such a long post lol