It might be an idea to get someone to sit in the back so they can hear the noise - sometimes its difficult to pinpoint the exact location of a noise. If the noise is from the back it could be a wheel bearing although they tend to rumble rather than squeal, if you jack up the back of the car, put it in 1st gear (to stop the car rolling) and release the handbrake and spin the wheel, it should be smooth running with no sound, as GOLFGL says, grab the wheel top and bottom and waggle it - it should have no play.
The CV joints are at the front of the car - power comes out of the gearbox and is transmitted to each front wheel by a driveshaft, to allow the driveshaft to move up and down etc with the car, a CV joint (sort of hinged joint) is fitted at each driveshaft and is packed with grease and has a rubber cover over it.
If the rubber cover (or gaiter or 'cv boot') spilts, water gets in and can damage the joint - this could give the symptoms you describe if the noise is from the front.