ATSOU

This is not related to Lexus but hopefully someone will have some idea about this. My dad has a 2000 Dodge Durango 4x4 that he uses to tow his big a$$ fishing boat. When he hooks up the trailer to the SUV, the back of the car is lower due to the heavy weight of the trailer/boat. Should we change the shocks or the spring plate (it's not coilover springs).....or something else? And where can I find those parts?

Thanks