Hydraulic Pump one Wheel Motor?

I have a '07 John Deere Z445 Zero Turn with a 54" deck. On level ground, it goes fine. Trying to go up a small grade in the yard, the left side won't go at all. This mower has a single hydro pump so I'm not sure if it is the Wheel motor or the pump itself. I'm fairly mechanical so I'd like to do this myself...just don't want to throw money at the wrong issue. Thanks,

Jeff

Hydraulic Pump one Wheel Motor?