Getting 12v isn't a bad sign, but not getting any amps would be... I had similar issues with mine once (was a bad battery ground); it would be just fine, until I turned the key to start... then the truck went dead. Shut the key off, turn it back on, and I had radio, lights, etc again. Turn to start, click. Dead truck.
Getting 12 volts through a bad connection isn't hard; but when you can't pull amperage through it, the relays kick out.
Have you checked the fuses/relays under the hood? (particularly the ECB-B fuse, as I recall).
Pull a few and check them; look also for corrosion around the legs and plug-ins. A number of people have reported this as an issue. Does your glow plug controller still 'click' and then kill the power to your truck?
Hook up a set of booster cables between batteries... positive to positive, negative on driver's side to alternator or AC bracket. That should eliminate the bad ground theory.