I have a 2000 Blazer ZR-2 that has developed a low growl in the trans/transfer case area and a marked decrease in mileage. The mileage dropped from 17 mpg in 2wd to 12 mpg. However, when I shift (push button) into 4wd (on a wet road) the mileage goes back up to 17 mpg. I have checked to see if it really was in 4wd on snow and all four tires turned. What's the problem?