1990 Chevy Blazer where do i start

full size blazer, 350/4sp manual, yesterday i smelled "burning grease" and the truck started vibrating above 50mph on hwy, also making a loud "whining" sound when off gas, also when going from stop i hear something banging into gear almost like an auto trans that is slipping...i checked the rear end fluid and is fine, nothing leaking that i can see...i am relatively mechanically inclined but little exp on 4wd vehicles, i would just like a idea on where to start....thank you in advance