D4C III Hystat – Won’t Turn

In need of some help here please... To start off, I just finished up an in-frame rebuild on the engine. Replaced radiator and thought everything was fine. Pulled it out of the shop yesterday and used it around the farm for about 4 hours. Pushed a little dirt, dressed up some areas nothing hard, just trying to break her in easy. SO, today a guy calls me and wants to buy my machine. Ok, I'm done with it mostly anyway. He comes out tonight and tries it out and about 5 minutes into the "test drive" it won't turn right or left! It moves forward and backwards just fine but when i try to turn right there is a grinding sound from the rear. Moving forward, backwards and moving the joystick to the left, there is no grinding. I pulled the rear inspection plate underneath the machine and I don't see anything out of the ordinary. I'm not real versed in the hystat operation so, does anyone have any ideas where I should start checking???? Thanks for any help,