2003 Chrysler Van

My brakes feel like they are constantly on. I can feel the lag when I step on the accelerator, especially going uphill. I have needed to have my brakes (and sometimes rotors) replaced about every 4-5 months, if not sooner. The driver's side rotor (I have rear drum) is severely worn and the brake pad is totally gone--it's metal on metal--and grinding horribly. The passenger rotor is worn, but there is still brake pad left. Once the tires were off, we could see that the caliper has the brake pad (or lack of) pressed against the rotor constantly. Would this be the caliper or would the master cylinder or booster have something to do with it? How far should the pad be from the rotor, in normal operating conditions, when not braking? I believe my rotor to be warped, as well, on the driver's side, as it periodically rubs with each wheel rotation. Why would the caliper hold the pad on the rotor on BOTH the driver's and passenger's sides, but wearing significantly more on the driver's side on pad and rotor. The rotor on pass. Side is more evenly worn down (about 1/8 inch), than the driver's side, which has deep grooves in it and at least 1/8 inch of wear. Would it be abnormal for BOTH calipers to be faulty, or would one being faulty cause the other one on the other side (over time) to become faulty as well? Also, does the whole caliper assembly have to be replaced, or is there a smaller part in the assembly I could replace or an adjustment that can be made to keep it from holding the pads on the rotors? Also, would this kind of thing cause my engine light to come on since the engine has obviously had to work so much harder, or is that another problem altogether? Thank you!