From what you have described I would check for a fuel blockage very often fuel starvation will cause the engine to drop to tick over revs and won't rev up, check stack tube from fuel tank for blockage,
Check any banjo bolt connections debri can collect at these points, if your truck has a hand primer pump situated over the fuel filter and the revs fall to tick over speed observe if the primer button has pull down this is a good sign of a blockage. I always think its worth checking for a blockage before moving onto the injector pump.

Regards Titus