Go to the local Auto Zone and get it scanned for any codes. They usually have an ODBII scanner and will scan the codes for free. If you have an p0216, p1688, or a p1689, it is probably a dying VP44 (Injection Pump). With that mileage on the truck, I will guarentee that the lift pump is dead (and has probably been dead for the last 50k), causing a failure of the VP44.
It could also be a CPS (Crank Position Sensor) or APPS (Accelerator Pedel Position Sensor) also refered to as TPS (Throttle Position Sensor).