Mark, our stick incident was also very ryhthmic, and occured so suddenly. Ususally, but not always, when a part begings to go south, it starts off as a "what the hell is that noise, and it never gets any better. I would jack it up, remove the wheel and do a close visual inspection and rotate all of the drivetrain by hand to see if there is some trail debris hung up somewhere.
Post ride inspections on our RZR have produced rocks, sticks, plastic water bottles, beer cans, deceased minnows & crawdads. Keep us posted!