my bet is he has an a 4g64 motor that is tier 2, his actual problem is either "wobble" in the crank sensor wheel (mounted external to the crankshaft pulley) or has the distance from the wheel to the sensor pick up just a -little- out of round (only 2 thousandths of an inch). you MUST use a non magnetic (brass) feeler gauge to measure, and you MUST mark the wheel at 12, 3, 6, &9 o'clock, and check the gap each place [remove all spark plugs and turn the engine by hand, check spark plug gap before re-installing the spark plugs]. Cat has/had a special "cross" that will hold the wheel in place as you tighten up the 4 bolts that hold it in place [if you work for an MCFA dealer the Service engineering department will loan your dealer the cross, I think]. There is not a lot of adjustment you can do to the pickup sensor, but the wheel itself can get out of exact position when it gets hit, (and on the 15s the wheel was so close to the frame, that hitting a pole would cause the wheel to hit the frame when the motor rocked, but even if it was not hard enough to damage anything else [other than paint], the scrape on the wheel was enough to cause the symptoms you describe.
These trucks would have mitsu 4g motors and the Impco spectrum LPG system, and were sold only in the 1st half of 2004.