yamaha grizzly 660 rear suspension?

I have a 2004 grizzly 660. I use it mostly for work around the property we live on.. pull a small trailer for trash, lawn work, etc. I do some "joy" riding on it but not alot. I am a large guy. 6'5" tall and weigh around 380 lbs. I have already set the adjuster on the rear shocks to the stiffest position. It still sags with me on it and the trailer hitched to it.. especially under accelaration... i am looking for options? New shocks? but how would i know if i'm buying something stiffer than what is already on it... new/different springs? any help or opinion is appreciated.. Thanks