I wouldn't do rear disk without also doing front. Fortunately disk on the DRW front has been done enough that you can follow someone else's approach rather than start from scratch figuring it out.
I'm not sure you have a problem if you can find the same rear I've got under mine. Its enough wider than stock that spring clearance should not be an issue.
The only question is brake fit, and while a 16 inch dually wheel might not go over the brakes the 19.5 certainly would.
I'd suggest some measuring rather than let my confidence replace reality, but since you have the 19.5 wheels you can measure them before going shopping for a rear.
