Any Zero-Turn Mower Experts?

I'm working on an '06 (If I remember correctly) Exmark Lazer Z 72" mower that is having hydro issues. on flat/down grade it does fine, but going up a hill the left side shudders and bogs really bad, but doesn't make noise. I'm suspecting wheel motor, but there is no debris in the oil tank and the oil looks fairly clean. The PO claimed all the hydros were gone through and redone, but of course no paperwork. My thought would be the pump would howl if that were the issue, but I'm at a loss. I know the correct way is to pressure check the pump, but I'm guessing others have had this issue before. Any thoughts?