First thing to check would be the condition of the drive belt(s) for the hydrostat drives. If that is not the issue,then you are looking at some major repair work, since it doesn't move forward could be an issue with both drives, but seems odd since they don't normally fail at the same time.
Even working on them yourselves, you are still looking at $300+ per side just for parts. And the amount of cleanliness required to have a repair that last for any period of time is paramount Dust and dirt contamination during the repair process will kill a rebuild hydro in short order.