I have a 1991 dodge d150 5.2 the oil pressure starts on low on the gauges when it is first started and when your driving it goes up to the middle but if you stop at a red light it drops all the way back down and it just started smoking real bad when the oil pressure is low , but when driving there is no smoke , I am thinking the oil pump but I am not sure

Think sludge. Start using full synthetic engine oil, to safely clean the engine. You likely have not been changing the oil and filter often enough. Oil pressure normally drops because of wear to journal bearings, not because of a bad oil pump. Be prepared to rebuild the engine, if necessary.