It sounds like the rear u joint, BUT while you have it off I would change the middle one too because one usually follows soon after the other.
Does the squeaking get faster as you take off? The hotter that joint will get the louder and more noticeable it will get.
You may also want to check the drive shaft to see if there are any dents in it as well, having an out of balance shaft will make it shutter and pull on one side of the joint causing it to do that as well, but usually when you here it, then it is too late and you are about to be left on the side of the road.