1994 dodge ram 318 back firing prob plz help hurry life deepends on it :P?

hi i own a 1994 dodge ram 1500 318 it backfires and miss fires and only goes about 20 mph when the gas is half or below when its above half it runs like new it gets up to 80 mph no miss no shakes no nothing just when it gets below half way it shakes and all donno if its fuel or fuel filter fuel pump the fuel pump makes a noise when it below half way mark plz help its driving me nuts its been doing this for the past 3 months im stuck in the house no way to go anywhere quick im hoping its a eazy fix (fuel filter or drain the gas) hoping not timing chain or something worst