Well, as much as I hate pulling up an old thread, my 99 Durango Slt 5.9 is suffering from this same problem. Mine is all stock. It started backfiring a couple months ago, and has slowly gotten worse. I replaced the dist cap and rotor and have replaced the plenum gasket. It got new 3923 plugs about 8000 miles ago. My next step is going to be a leak down test and new plugs and wires. Just wondering if you found a resolution for your problem?
Thanks.