So my 05 Xterra 4 by 4 is sagging real bad when I have my 03 Supra Launch 22SSV boat hooked up to it. The Boat and trailer weigh aprox. 4500 lbs....I really don't want to give up the truck, the power and stopping of the trailer is no problem, just the sagging has me worried... The trialer kinda sags to the front and my rear end of the truck saggs in the rear.....I was told that air bags might help, but I can't find a web site...Can you guys help, please....

Sounds like you simply have too much tongue weight. I'd look into getting the trailer axle repositioned a little forward to shift some weight off the tongue before I'd modify the vehicle.

I have a dual axel trailer...How would that be cheaper than getting air bags...