Did you clean ALL the rust and dirt off the hub when you replaced the rotors? A small amount of dirt or rust back there will give you excessive runout, and since you don't use the rear brakes much, you won't notice it at low speed.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!