Plug in the block heater.
The description you are giving sounds alot like when the glowplugs are off; you keep cranking and the truck almost starts. You keep doing it in 30 second "blocks" to let the starter cool, it will most likely start.
The compression (from cranking) is heating up the cylinder and helping combustion, but nowhere near as much as glowplugs.
The glowplug relay is on the top of the motor, just left of the fuel filter location - looks like an old ford fender-mounted starter solenoid. You should check to see if you're getting voltages for the switched post when you first key-up. Also, on a good state of battery charge, your OEM dash-mounted voltmeter will drop to about 8V.