It not just the shock length, but the eye on the end of the shock that mounts to the rear scissor is shorter og the 15 vs the 16 shocks, and wont fit, the shock hits the scissor.
But all the three other shocks fits, fronts and front track shock.
But the Fox RC2 burandt edition shock for the proride will fit in the axys rear susp, but it is shorter and causes a loss of about 2.5 inches of travel measures at the end of the tunnell. I am running it now, front end comes up easier but the ride is soooo much better than the stock walkers with the reservoir that comes on the pro.
Polaris really ought to offer some better shocks as an option with these sleds, just makes it a much better ride all in all.