Its just a thought, but try checking the pcv valve. I had a similar problem with my 2000 ranger. I started by replacing all 3 O2 sensors. Nothing. Then i replaced the fuel filter and pcv valve. Also i cleaned the MAF sensor. After driving it a bit, the black smoke cleared up and the driving gradually improved. The pcv valve looked like it had chocolate syrup in it. I sprayed some throttle body cleaner (safe for fuel injectors) into the valve and it seemed to help. Pretty cheap parts, so it couldnt hurt to try. | 
