GMC Repair: missing? backfire?, air induction system, gm dealership

Question
Ihave a 1996 gmc 2500 4x4 with 5.7 vortec the truck has 245,000 miles on it i was told when i bought it that the engine was replaced 40,000 miles ago. the truck runs strong idles fine. the problem is that when i get to 50mph or more and let of the gas a little it fells like it missing and you can kinda here it want to back fire, when you run it in drive you can really notice it, when running in od drive its not as noticable. when you floor it at this speed it dosent miss a beat. i just put new plugs,wires,cap,rotor hoping this would fix but it didnt. no engine lights on or anything like that. maybe cat. converter or fuel filter. im pretty mechanicaly incline. thanks for your help

Answer
The first thing I would suggest is a fuel filter and an engine decarbon.

The way that I like to do an engine decarbon, which is how I do it on my 1998 Tahoe, is to go to your local GM dealership and get a can of GM spray top engine cleaner, and a can of liquid top engine cleaner. Take the air induction off of the truck and start it, spray the first can into the throttle body making sure to manually work the throttle to keep the engine running, and clean the throttle body. Next, pour the liquid top engine cleaner into the throttle body, again keep the engine running, and when the can is about 3/4 empty dump the can in and let it stall the engine. Put the air induction system back on the truck and disconnect the battery to clear ant codes set by this and let it sit for 30 min. Then start it up, this will be somewhat difficult because it will be flooded, and drive it. Try to keep the RPM's high for a while. There will be alot of smoke coming out of the tailpipe, drive it until the smoke is at least almost gone and see if it is better. If not get back with me. The top engine cleaner should be about $7-10 each.