My question is if you have a 3 channel rear what do you have to do to make it a 4 channel like on a S60 or 12 bolt?

 

axles and backingplates like a 10bolt i assume?

 

Yes. 10 bolt

 

Need to add tone rings to each axle and the magnetic sensors on the brake backing plates (along w/ the appropriate harness for the 4-ch mag sensors). 3-channel rears will not have the tone rings on the axles (they use a single tone ring on the ring gear), which are needed to send in to the fab shop building your new rear axle. I am not sure if the brake backing plates for a 3-ch rear has the provisions for mounting the mag sensors.