Why one drive side has less power than the other?

I actually have a Craftsman Z6900 zero turn mower, but that wasn't an option to select.

My problem is that the drive on the left side is not producing as much power as the right side. On level ground it operates fine, but as soon as I have to go up a hill I have to fully engage the left side (and I hear a groaning sound, like power steering in a car when the fluid level is low) but the right side can stay just partially engaged and provides plenty of power. I have read that my mower has a hydrostatic transmission, but I have no idea if this one is serviceable or how to do that if it is. Any ideas? Mahalo.