I have a 1987 Chevrolet suburban v10 4wd with a 351, it's been experiencing a problem that seems to be when it goes to shift into a higher gear it's as if it slips into neutral, the revs rise quickly with no gain in speed. Flooring the gas seems to get it to up shift but at that point it is revving way too high. Putting in in 4wd seems to help slightly but does not completely alleviate the problem. Would this be a transmission or transfer case problem and should I replace either? I changed the transmission oil which hadnt been replaced in a long time and the oil that drained out was slightly black.