Why does Whirlpool fridge leak water from freezer down to the fridge?

For at least a year, the freezer drips large quantities of water from the back of the freezer down into the top shelf of the fridge. I have to leave 2 large tupperware containers on the back top shelf to catch the water and dump it about once a week. It seems the back of the freezer gets clogged with ice and that spreads along the whole bottom of the fridge. We once emptied everything out and used a hairdryer to thaw everything out and that worked for about a week until it all started up again. I have read similar complains on other websites and no one seems to have a solution. Any ideas? The fridge is only about 3 years old and I don't want to replace it. Thanks!