I had a similiar issue. Put a test light on the crank fuse. It should light when you turn the key over. If it does not, then you have a bad ignition switch or it is not indexed correctly.
If it is good, pull the starter relay, and put the test light in the Forward Passenger side hole for the relay. Turn it over again. If it fails to light up, the problem is the NSBU. This was the case for me. I had a pin pop out slightly, and with the switch installed in the truck, I couldn't see it. When I pulled the switch out, it was obvious. There are 2 pins on the NSBU that could cause this same problem. You would want to check a diagram to figure it out. My LB7 has a 2 plug switch, so it is a little different.
If your test light has power on the starter relay and the truck doesnt run, then check your INJ A fuse under the hood. It sends power to the AFT Passenger side hole on the starter relay. If the NSBU switch says that the tranny is in neutral, it energizes a contact in the starter relay which closes the loop between the INJ A fuse and the starter.
If everything has worked thus far, and you haven't solved the problem, hook up a continuity tester (usually a function on a multimeter) to the Forward Driver hole for the starter relay and put the other end on the exciter wire on the starter (the small post). You should have a continuous unbroken wire here. If it does not have continuity, trace the wire from the fuse box all the way down the front of the motor to the starter.
If all these things check out, the only other cause that I've heard is that the ground has been burnt out in the TCM and you need another one...that has been LBZ trucks that I have heard that for though.
Good luck and if you have any questions Dustin can get ahold of me or call me at 757-477-2182
Thanks,
Robb Dearth