1996 Jeep Cherokee smoke coming out of ? tail pipe/back end

1996 Jeep Cherokee 6 cyl Four Wheel Drive Automatic

I checked all fluids and they were fine. I do have an oil leak somewhere according to Walmart oil change people. No major losses or leaks on driveway. Just replaced starter (a week ago). Was driving fine, then there was a smell and hot air coming in from vents. I checked belts, them seem tight and all there. Help please? Possiblilities???