there are pre made kits to convert your 9" drum rear to disk. Its been awhile since I priced them but I believe its around $600 or so. Cheaper than the front disk conversion kits.
They did make disk brake 9"s. But I am not sure of the width that you would need.
check out www.ultrastang.com gobs of info that ultrastang has compiled. Click on the 9 inch rear end. Link on the top of the page.
I put some on my 9" from Currie Enterprises (there quality sucks on axles and diffs) only because they are OEM Ford Explorer disk brakes in a kit form which are very good. (the after market parking brake cable is junk but your original cable will work) I had "SS" brand rear brakes on my 8" and they really sucked by design, quality and customer service!
I did the UltraStang kit with the Lincoln Mark VII parts. So far I am $240 into it and have everything I should need--rotors, calipers, brackets, new master cylinder and a 4wdb distribution block. I did luck out and found a really really really nice set of donor rotors, calipers and other bits for $60--but you might too.
I saved so much money versus what I had "planned" to spend on a SSBC kit I splurged and am upgrading the fronts with MustangSteve Cobra stuff. That will be cheap too as I will sell my original factory disk setup once done.