Have an old Sub-Zero refrigerator (model 2811) with a clogged or frozen drain line which causes water to collect in the bottom of the refrigerator every couple of weeks and drip out the front after the tray in the back overflows. Assume it's frozen. How do I unfreeze without having to move all of the food and then keep from refreezing. Thanks for your help.

Your drain tube may be stopped up with ice at the upper end because it drains too slow because it's stopped up at the lower end in the evaporator pan under the unit at the floor. It can get dust and mold in it. Once you get the ice out at the top a little pressure with a turkey baster will usually clear it out. Flushing it out with hot water and clorox may help. Make sure it drains quick enough to prevent refreezing. . The drain should be located below the evaporator coils on the lower back of the freezer.
Take a bare copper wire and wrap it around a fin near the heater and put the other end a few inches down the drain. The heat convection may prevent it from freezing.