Sounds like your starter solenoid is about gone....or you have something shorting out. If I am understanding what you wrote correctly....I certainly think that you need a new starter. Did this come on all the sudden? Does it end up starting the truck?
If you want, you certainly can get it tested first and confirm that it is drawing way too many amps.....and if you can't find a wiring issue or any other reason for it doing this.....the only thing left to point the finger at is the starter.