How is your exhaust doing these days? if you own a Bronco, the exit of the exhaust should be behind the rear passenger side's wheel, facing the side, not the rear. If by some coincidense it's facing the real that could be the cause of the smell (you still have some other issue to work out first). Bad combustion of the fuel (if the smell actually comes from the exhaust) could be caused by a dead cylinder or fouled spark-plugs.