wheel bearing
 

Is it possible for a bearing to make a grinding noise in one direction, but if it turns the other direction not make any noise at all?

I dont think so I think your problem lies in your pinion to ring gear interface. That is why it makes noise in one direction and not the other.

But who am I:o

The fact that it does it under acceleration and not coasting (you're loading and unloading the teeth), sounds like a ring and pinion, but the high pitched squeal sounds more like a wheel bearing or perhaps a pinion bearing ??
look at the teeth on the ring and pinion ... they should have almost a polished look.

yeah... they look slightly polished.

Well i'll admit i'm pretty much the village idiot when it comes to positractions ... maybe the plates or clutches in the posi squeal like that ??? One of you posi experts out there help us out !! Do they squeal like a stuck pig when they go bad ?