4wd major issue

[TALL00TAHOE]

BOUGHT MY 2000 TAHOE ABOUT A YEAR AND A HALF AGO, WHEN I GOT IT HOME I CHANGED ALL FLUIDS SPARK PLUGS AND EXT.. WHEN I WENT TO CHANGE THE FLUID IN THE T/C IT WAS LOW (ONLY ABOUT A 1/4 OF A QUART) AND REAL BLACK, PLUS IT SMELLED LIKE BURNT UP BEARINGS. WELL THIS WINTER MY FEARS ARE CONFIRMED, NO FOUR WHEEL DRIVE HIGH OR LOW AND MY AUTO 4wd SEEMS TO WORK SPERATICLY. LOOKING FOR SOME INFO ON HOW THE AUTO AND STANDARD 4WD'S WORK TOGETHER AND MAYBE A DIAGNOSIS ON WHAT WOULD CAUSE THEM BOTH TO GO OUT. IM NOT SEEING ANY LIGHTS INDICATING IT IS ENGAGED OR IN NEED OF SERVICING??? IM JUST NOT REAL COMMON WITH THE NEWER 4WD SYSTEMS AND HOW THEY WORK. THANKS IN ADVANCE