Master cylinder, or rear wheel cylinders. It wheel cylinders aren't leaking, then it's the master cylinder. I've replaced the whel cylinders and master cylinder before. It's really easy. If it the master cylinder though, you will need to take it to a shop that has the special tool used to bleed the abs modual. I tried to bleed mine myself, but the haynes manual was right. You need the special tool. That assuming that you have 4 wheel abs.
2003 Ranger FX4 level II. 3" BL, 33x12.50 GT Radial Mud Claws, Custom front and rear bumpers with winch hookups, custom flatbed, with hi-lift, and full size spare mount, 12,000 lb winch mounted under bed, Electric fan, and of course a flowmaster.