That was an option on the GMC trucks. The theory behind it is that the I beam axle resonates at a certain freequency. The dampers resonated at the same freequency but opposite of the axle canceling out all road vibration. I had a pair on my first 59 gmc. I didn't notice any substantial difference with or without them.
John
I would say it also adds more weight to the frontend , these things are heavy!
