'05 Blazer 4x4 issue
 

My friend's '05 Blazer is having issues whenever 4x4Hi is engaged. Everything is fine until you hit about 60km/hr or so, when it begins to feel like the entire truck is vibrating violently. If you drop it back into two wheel drive, everything reverts back to normal. Considering I'm quite unfamiliar with four wheel drive in general, and this truck in particular, I was wondering if someone could shed some light into why this is happening and what may need to be done in order to fix it.