Put in a freshly charged battery and then find out the following:
What is your OI reading for voltage when you are tring to turn?? You may be spiking out the system with using only two drill motors for 4 wheel drive?
If it is reading anything less than 6 or 7 Volts then I suggest you either redesign your tranny

or for goodness sakes, make some wheel adjustments like stated above!!