I haven't yet, but its on my list. I haven't done much to my truck as of yet other than trying to get my motor running nicely. I've almost got enough money saved up to do disc on all 4.
http://www.ebay.com/itm/chevrolet-c1...item35c2b974f2
Price comes out to 1150 after shipping for 4 wheel discs, power booster, and spindles(drop or stock). All you would need to do is build some brake lines which aren't to terribly expensive and your whole brake system would be brand new.
Even if you don't do the rears then I would do the fronts to disc for sure.