3.9 backfire problems

I bought a dakota sport 2wd 3.9 that had been sitting in a yard for 3 years. It backfires once i let off the gas, if i rev it passed 3000 if i keep under 3000 i don't have a problem. apparently it has had this problem for a while. gas apparently pooled in the muffler on the highway and ignited causeing the loss of muffler. current exhaust is 3 inch pipe w/ no muffler. i replaced the O2 sensor and put a new air cleanner in but the problem is still there. i think it might be the fuel pressure regulator. Does anyone have any ideas on what the problem might be? thanks for any feed back

That's revving under load? Or just revving it in neutral?

thats in nuetral and when i am driving it as well