So first have you adjusted the preload on the shocks all the way up ? To carry both your weight .

Rear Shock Absorber Adjustment - Yamaha KODIAK 450 Owner's Manual [Page 46]
Yamaha KODIAK 450 Manual Online: Rear Shock Absorber Adjustment. The spring preload can be adjusted to suit the rider's weight and riding conditions. Adjust the spring preload as follows: To increase the spring preload, turn the ad- justing ring in direction a. To decrease the spring...
