Uneven heat with forced air furnace?

I have a 2100 square foot house (one level) that heats unevenly with an electric forced air furnace. The temperature difference begins at the furnace. One side of house is too warm, center is right temperature, and other side of house is too cold. Furnace is a 5 ton unit which techs have replaced the blower with a 4 ton blower to try and even heat. Didn't work. Any suggestions beside replacing unit as I think unit is too big for house. Work done by company who is notorious for oversizing furnaces when house was built in 2000.