I have 45,000 on a 99 XLT. I'm in AWD 99%+. Lately I've been getting a kind of scraping and roaring sound when accelerating at 40 to 60 mph. The sound will stop if I put it in 4WD. Has anyone else had any transfer case problems with the AWD? Is this a sound of aging or a death rattle. I don't want to go to a Ford dealer shop or transmission shop for "exploratory" work if I can avoid it.