i have a few ideas, you could always check it out.
Are you sure your only getting 6v AT the pump ONLY? Stupid question, but its best to start with the simplest cuases 1st before going more in depth. Check the battery terminal JUST to be sure. even if the truck cranks over fine. you never know.
another thing, are you checking voltage at the pump while its connected, or at the connector while the pump is not connected. If you are taking it with the pump connected, theres a chance that the voltage is fine, and the pump is siezed up. If thats the case, the pump would be drawing maximum amperage, and given the size of the wire, or the amount the relay or fuse is able to carry, it could technically drop the voltage of the fuel pump circuit that low, possibly even without blowing a fuse.
then theres another thing, check the relay connections, and scan the wire (to the best you can, i know it can be tricky) all the way to the pump, and check for any shorts to ground or any partialy opens in the wire. The relay may also be bad, cuasing only a little voltage to slip by, but ive never seen this happen. Its unlikley, since its typically a part that fails complelety when it does go out. but it can happen, and given your readings, i would check it out to be sure.
I would suggest after proving that the fuel pump itself isnt bad, is to disconnect the battery and disconnect the pump connector, and take ohm readings from the fuel pump postive wire to ground at all the pionts you can running working towards to the pump to see if it is shorting to ground anywhere, even if you get a partial reading, that is bad. You should see infinate ohms (open) all the way to the pump.
Good luck, this problem is rather simple,and after taking the checks i menchined,you should narrow down the cuase in no time
