I have the same problem with my toybox. Because of the need to be able to haul inside the rear of trailer it has a heaver toung weight when empty. I checked the 03 Navigator, It seems it has the same suspention. The only difference is that the shocks have airbags connected to it on top. I went to the parts department and the airbags are sold sepretly from the shocks. The 03Expy has a place for a switch in the back for the air susp. I was possibly thinking of buying the airbags and placing it on the shock and adding a compressor and switch to manually lift the rear when towing. The only problem is that at $130.00 an airbag its a pretty steep investment on something that I am not sure would work and it is a special order part so I cant just take my shock in and see if it fits.