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