New master cyl is like $90 if you do it yourself.
There is a brake biast adjustment on the plunger that comes out of the brake booster, this adjusts the front to rear calibration, however I cannot recommend that you adjust it, the Ford books say never touch it, the proportioning is supossed to be self adjusting on your truck. We have fooled with this little sucker a few times because our truck originally had drums, so getting the proportioning correct is impossible without the correct master cylinder (too lazy to install so far). Brett wrote an article on how to convert to rear discs, you can find it on the board, this will explain more about adjusting the front/rear biast.
My rear discs drag at all times, this is due to a few things, I am still using a BII master cylinder which is designed for rear drums, I have deleted my RABS from the truck, and Ihave the proportioning set as good as I could get it which causes the rears to drag slightly. So I go through a set of rear pads a year, thank goodness for Autozone's lifetime warranty!!!
The plan is to install a 95+ Explorer master cylinder with the correct front and rear disc proportioning.
the rotors are a PITA to get off, first make sure your parking brake cable is loose.....then you unbolt them I believe and then they pound off with a rubber mallet (I have never taken mine off so I;m not sure but I do know they are a PITA) somebody else can chime in here on this one....
the rear rotors are thin to begin with and with excessive heat they will warp easy and with the pads always dragging they will wear out quickly, so to me it does sound like you are due for a master, calipers, and rotors. Thats the bad news. First you can call around to some performance brake places (Willwood or Brembro dealers) and see how much it would cost to have the problem diagnosed.......
You can tell by looking at the wear on the rear pads if the brakes are dragging at all times (like mine), the rear of the pads will be more worn out then the front, sothe wear on teh pads is greeater towards the rear of the truck....this points to calipers getting hung up on their slides...........