Wait, so all I had to say was that the truck won't start? That would have saved me a lot of typing.

To provide additional information. The starter gear spins just fine and most of the time will engage with the flywheel and crank the motor until it either starts or I release the key. I have cranked for as long as about 30 seconds with no problems. No slipping or problem once engaged. However, occasionally, the starter gear spins and just grinds against the flywheel so it is not engaging or not extending far enough. If I get the grinding I immediately release the key to prevent any wear of the gears. The starter is new and that gear is fine and the flywheel does not appear to be ground down or damaged. Could this be a problem with the 6v solenoid or is it something else? Thanks.