I have same sled. My suspension was the same as yours and felt sluggish on the rebound with 1500 miles on it. Usually it would sag about 2 inches and not return to full lift.
Last month I decided to refurbish suspension, which included new bushings, sand blasting arms, painting them, and rebuilding shocks. A local guy out of Missoula recharged shocks and replaced some seals for about 20-25 per shock.
Results were amazing and the sled rides like new again. If you push the rear tunnel down it is an automatic quick return to full. NO SAG at all and a consistent rebound every time.
Shocks would be your best start in my opinion as they recommend recharging and inspecting these 4 units every year.
Good luck and let me know if you have any other questions as the process is still fresh in my head.
NM