Backfire under loading going up hills in second and third gears?

My truck recently started back firing and rear end vibration/ hopping while going uphill at a steep grade. Only time it happens is put hill. Not on start nor flats or downhill. After warming up it idles around 500 RPMs when its set at 700. After driving for 5-10 minutes idle sits at 700 RPMs. Also smell of gas/oil on startup through heat/AC vents into the cab then goes away after 2-5 minutes.