There could be a problem with transfer case, the front differential but, the tell tale sign of a worn CV joint is kind of what you describe.
Is it a clunk, knock or vibration? If it's a knock or clunk, go to an open parking lot make drive in a slow, tight circle, first to the right, then to the left. If it's one CV, you should only hear the knock in one direction or the other (Unless it's both).
Remember, even if the boot looks good doesn't mean the join is good also.

Not to say it's not, but, generally you'd only notice a bad belt at higher speeds and if it were bad enough to feel at slow speeds, you should be able to see it too.