So I have a 99 silverado 4x4 5.3L LS. When I'm in 4wd I've been noticing a howl from the front left side of the truck. Thinking it's coming from the transfer case or a cv joint. Also noticing a clunk and a jump at time when I try to start moving. It likes to hop sometimes too. Any idea of what could be causing all that?

 

I'd maybe pull the fill plug on the front diff and stick your finger in the oil, if it's all full of metal and glitter it is likely toast.

Also check all loose front end parts for the clunk and wheel bearings.