Yep the axle shafts have to come out to get the backing plate off.
If you're wanting to do a rear disk brake conversion to make brake maintenance easier, go for it. If you're wanting to do it for better braking, save your $$$.
Converting rear drum brakes that are working properly to disk brakes won't improve your braking performance or make you stop any faster. I did the rear drum to disk conversion on my TJ years ago when I had more $$$ than experience with rear brakes and was bummed that there was absolutely no discernable improvement in braking afterwards. I knew there wouldn't be much, if any at all since the rear brakes only provide 20% max of the total braking, but I was hoping there might have been... nope.