Mick-De, the 84 XR and CR rear hubs are different, but not totally. I reckon you could get the CR hub to fit. The only mojor issue i see is that the CR axle is thicker than the XR, which would mean either altering the XR swingarm or maybe find a bearing than fits the CR hub (outer dia) and the thinner XR axle (inner dia). Put on a CR master cylinder up front and you might have some brakes on the XR!
Cheers
Mark