Backfiring while accelerating?

I just got the truck listed above Sierra k1500. It had been sitting awhile and was only driven to go hunting. It starts right up with no problem and idles just fine, when I start driving it'll backfire and skip and feels like it wants to shut off and is using way too much gas. I replaced plugs and wires and was trying to figure out where I should go from there. The exhaust also isn't run all the way, it stops about where the bed starts. Please help!