Axle flange out of square.

I am installing a rear disc setup on my 9". I placed the rotor on the flange, lugged it down, and setup a dial indicator touching the pad surface of the rotor. When I rotated the differential, the rotor showed 0.04" out of square. I then rotated the rotor relative to the axle and checked it again to see if the rotor or the axle flange was causing the problem. It's the alxe flange.

What should i do? Should I shim in between the rotor and the flange?

The proper thing to do is either have the axle straightened or get a new one. BTW the term is "runout" rather than square, it'll make more sense to the axle guys.