Have you tried making the adjustments with the wheels off the ground? There is a video online of a guy doing this with his Toro zero turn. The concept is that the drive wheels have absolutely no motion, even without a load while the tires are lifted off the ground. You simply block the front tires, lift the back off the ground and put up on jack stands. With the mower running (may need to disconnect seat switch), you pull you levers in from park and then adjust the hydrostatics so that both sit still. Then from here you can see how it tracks. Also, the only way to tell if it is actually tracking straight is to be on level cements and to have the throttle fu with both levers all the way forward. Also, check your bypass levers that allow you to push the mower and ensure that they are completely disengaged. Also check your tire pressure that both rears are exactly the same.