The lift pump will only run for 30 seconds if the starter is bumped. If you are turing the key to start it, it should turn on for like a half second while waiting for the Start Engine Light to come on.
This is kind of a pain, but it might help you figure out if you are having a problem with the lift pump: Replace your fuel filter (get it from a Cummins dealer and save over half that of a Mopar filter). Drain all the fuel out of the bowl and clean the bottom (if necessary). Put the new filter in with the bowl empty. Partially close the top - keep it loose enough so that air can escape. Bump your starter (do this by turning the ignition so that you get the lights on the dash on, then just touch the ingintion so that the starter gets to run for like a 10th of a second - as quick as you can get the starter to engage and that's it. Don't turn the key all the way to the off position, let it sit with the dash lights on. The lift pump should run for about 25 seconds (you should be able to hear it in the cab). It would be a good idae to keep an eye on the bowl to make sure fuel doesn't go all over the place. If you get fuel right away, turn the key to the off position - note - the pump will still run for a couple seconds even after you turn it to the off position. After doing this once, check the bowl to see if it filled up. If you don't get any fuel the first time, try it again. If you still don't get any fuel, then your lift pump is probably dead. Best bet if the pump is deal is to go to a Cummins dealer and buy a new one and do the repair yourself. It's a very easy job, and there is pletty of information here to guide you through the process.
Good luck