The ignition cylinder provides the ground for the relay circuit which energizes the coil in the starter relay and shuts the switch. It shouldn't need any movement of the truck to do so.
You can check the coil by supplying power to pole 85 and ground to 86. If you hear it click then more than likely the relay is good.
It seems to me, and I may be wrong, that a bad starter relay wouldn't cause the truck to die. Plus, relays usually aren't an intermittent problem. If it were me I'd the ignition switch and circuit, and the starter relay wiring would be suspect. Maybe bad contacts in the switch or a short to ground somewhere.
