100% agree with Beeutey,pull the rear tailshaft ,take it for a spin at that speed you reckon its the most audible.If that dont get rid of it try doing the same with the front tailshaft.Then dont jump to conclusoions,it could be the tailshafts themselves ,uni's u/s, bent tailshaft,outof balance tailshaft, handbrake catching,etc etc or a Tyre issue