Won't crank

Hey guys, I've got an 06 Ram 1500 2WD 3.7L. Crappy on gas mileage but I love the truck. I've got an issue though, it won't crank.

When I put the key in the ignition the lights, radio, etc will come on. But nothing when I turn the key to crank. Put the truck in neutral, still won't crank. The battery has 13.5V, and also voltage is going down to the starter as well. The posts and terminals on the battery have been cleaned and put back on, still won't crank. Temporarily, I've got in hot-wired right now to get it started but it takes two people.

I hooked up the computer dashboard diagnostic reader and there were no errors. I'm thinking maybe a sensor has been knocked loose, but the sensor, whichever one it may be, isn't loose enough to throw up an error? Or am I dealing with a Neutral Safety Switch issue?

Last question, can the oil pressure switch, if clogged, keep the engine from cranking? Wouldn't that have put an error code into the dashboard computer?

Thanks guys, love this forum.