You know there is this cool thing out now called the internet type in gm starter remote ford solenoid and TA DA problem solve....crowd goes wild
its a cheap easy fix if you do this and have same issue you have either
a starter to close and getting stuck on the flex plate for some reason
b low current to the starter
c remove starter attach to a 12volt source your battery and jumpers
work great.Fire that sucker up see if the starter drive snaps in and out
quickly.
d have someone try to turn the truck over wit a GOOD voltmeter at the
battery,you should get a voltage drop when you try to start.
Please try these out post the results and i will be more than glad to help
figure it out.