What White Max said. Its a known problem with these trucks. Mine did this for a couple of years. I finally plopped down the $16.00USD to buy a new cylinder. It literally is a 5 - 10 minute job. There is a hole underneath the ignition key cylinder. just put a skinny object (I used a torx screw driver) in the hole, turn the key while pulling out on the ignition key assembly. It might take a little doing, but ideally it pulls right out when you get a feel for it.

I highly recommend fixing it. The couple of times I let others borrow my truck; they accidentally left the ignition in the "ACC" position which caused the battery to drain.

The cons of this is you will have a separate ignition key from your door key. I thought it would be a major inconvenience, but it actually has some advantages. You can have them "keyed" to be the same but it costs money.

From a safety standpoint, there have been some accidents caused by this problem. Not too long ago a 5 year kid started his grandmother's truck and drove it into a swimming pool!