This is a bit difficult to diagnose from the information you've given, but here are a few things you can try to narrow down the possibilities.

First, does the noise only appear when the car is moving?  If so, it's definitely something to do with drive shafts or wheels.  If you hear a noise when the car's stationary, check the steering pump drive belt and the steering lock stops - full lock means maximum load on the pump and if the belt is at all loose the pulley will slip and squeal.

Assuming it is only present with the car moving, is it there on both locks left and right) or just one?  First make sure the tyres aren't fouling the wheel arch somewhere, especially if you've recently had new tyres fitted.  You can do this with the car standing still and the wheels on full lock - check the wheel arch shields in case they haven't been properly refitted or are damaged and fouling the tyre.  The fact that it only happens when you're reversing is a bit of a clue because the wheels will move slightly towards the rear of the wheelarches in reverse due to the play in the suspension, so if there's very little clearance when the car's not moving it could be rubbing when you reverse.  Then check the CV joint rubber boots - they're doing a lot of streching and compressing as the wheel turns at full lock.  

Hope that helps.

Tim