I believe wiper motors generate about 12-13 ft/lbs of torque, so if you have a small spindle to use instead of a winch, a wiper motor may very well lift a 50lb load...slowly (Cap'n Jack is on to something with using a lighter door).
Something I'd suggest is counter-balancing the door.
If the door's sliding vertically, is there room to just add another rope to the top, run it through a pulley, and hang a 25lb weight plate on the other end? You're effectively reducing the weight of the door by 25lbs, and the wiper motor may be able to move that load quick enough to make your prop work properly (heck, add 60lbs to the other end of the rope, and make the winch/wiper motor PULL the door into place - so when you release it, gravity will simply pull the door up and out of the way as quick as you need).