I've got a '94 4wd 5 speed that suddenly started a bad vibration from 15mph to around 45mph. It feels exactly like I'm trying to start in third gear instead of first. It only happens when I'm in gear,( any gear ) and ceases when the clutch is engaged or i'm coasting in neutral.
I'm about to roll 150,000 on this well-maintained, never wrecked vehicle.
Any ideas what the problem could be?

 

No weird smells?