Bad injector pump?

[GROBBINS]

I am new to TDR and to diesel engines, so please bear with my ignorance. I have been having a problem in the past several months that has been progressively getting worse. It is a 97' with the 12 valve, auto tranny. It started when I was backing a loaded trailer, I let off the gas and the engine killed. I didn't think anything about it, assumed that it was bad fuel, replaced the filter and thought I fixed it. Then a couple months past and I started noticing that if I floored it quick and then let off the gas the engine would immediately kill. Now, if I pull up to a stop light with the tranny in drive, the engine rpm will drop below 300 rpm to almost 0, cycle between the two for a few seconds and then kill. It won't start until I give it a little gas. Once I put it in drive it starts the rpm fluctuation again until I am on the gas and accelerating. At every stop sign or light, I have to put the tranny in neutral to avoid the engine killing. If the A/C is on or my steering wheel is cranked its hard to keep the engine running while stopping or turning. I have starting to drive with two feet just to keep the engine running while maneuvering. I would love any information that would help me make my truck more reliable and not an embarrassment. Thanks a million! Gabe