If this is the same truck as the one where the Cam & Crank timing is OFF.......this would make sense. If the distributor spark is retarded (firing too late), raw fuel is getting into the exhaust and could be causing backfires. If the distributor spark is advanced too much, you can be getting detonation or spark knock.
If the cam & crank timing are not set properly, then the distributor spark timing is more than likely off as well......and could be too advanced or retarded.
