My 2000 Blazer ZR-2 developed a low growl from the transfer case area (sound kind of like deep-lug tires, but not as loud) and I have noted the mileage has dropped from 17 to 12 mpg in 2wd mode. However, when I shift to 4wd, the mileage comes back up to about 17 mpg. I obtain the mileage reading from my on-board trip computer. In addition, I determined that the 2wd and 4wd modes are working correctly by testing them on snow back in April. What's the problem? MKB
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Sounds like the transfer case vac switch is leaking and partially engaging the front axle

It's about $17 at Orieleys Dorman part# 600-500