You have to get a weight with the WD unhooked also. As it would seem too you need to get more weight on the front axle...
See here..
http://towingplanner.com/ActualWeights/TravelTrailerCatScalesThe trick is to restore front to original weight without going over..
See mine here.. I did this all by measuring the wheel wells when setting up the WD hitch. I then went to the scales and just happened to be right on the money..
Truck only
Truck without WD engauged..
Truck with WD engauged..see how I restored the front axle.