I went through the same thing with my '01 ST. I discovered that the disk brake 4 door and Sport rear ends are identical to the drum brake rears on the 01-02 ST. I bought a rear end out of a '00 4 door Explorer w/4:10's, from a local salvage yard, pulled the axles and swapped over the drum parts from my old ST rear. You should pull the axles to install new bearings and seals anyway, so it's easy to swap out the disk parts and 5th shock parts you don't want.
Seriously, stay away from parts finder services. You'll pay through the nose and if you have my luck you'll pay twice what the rear is worth and it will be junk. Good thing I paid with a credit card, disputed the charge and got my money back. The seller wanted me to return the piece of junk they sent me and expected me to pay the return shipping. I told them no f'in way and my credit card company agreed. I ended up taking the piece of junk to the recycler and got a few bucks for it as scrap.
Edit:
Some here will tell you you should just keep the disk brakes on the used rear diff and will say they work fine without brake system mods, but I don't believe you can do this w/out changing the master cylinder and/or proportioning valve (maybe even the ABS module) because rear disk calipers require more fluid to operate than wheel cylinders. I've installed rear disk kits on late 60's mussel cars with disk/drum combos and have had to change the master cylinders. Besides, there's nothing wrong with rear drum brakes if you know what you're doing.
