I have a 1990 FCS3 turbo II jspec I had it out yesterday and was getting on the gas coming out of some tight turns then it happened. I'm not sure but I think something's happened in the diff. if I turn left I hear and feel nasty gear noises and the car shudders BUT if I hold in the clutch and coast through a left turn it's fine.

Is this something other than the rear diff needing a doctor?

 

Yes, it's bound to be the diff. Jack the rear of the car up, rotate the wheels and listen for grinding.