have you swapped side to side. maybe your other shock has a valving issue causing it to act stiffer.
I assume your mountain sled doesn't have a sway bar?
if it does, sometimes these twist out of sinc, making one side loaded and one side not all the time on even ground.
pull your springs and set them beside each other, that will tell you if your spring is sacked.
while spring is off, do a drop test. Nothing fancy but will tell you how the two compare on compression and return. you will see if there are any dead spots in the shock as well.
Other than that, loose parts on one side and not the other, as in bushings, or maybe a mount is broke in the frame, making the sled have to move more to take up the slack and start using the shock?