Austin Western Super 301 Motor Grader – Jumping out of gear

Austin Western Super 301 Motor Grader Model: 201-986A Serial: 87373 The customer states that while operating this machine down a grade it feels like it jumps out of gear and starts to free wheel. I got it to the shop and started looking the machine over, I am thinking that the transfer case or it's controls would be where the problem would likely be. The controls are electric over hydraulic and I traced the wires back to a 3 position toggle switch(unmarked) on top of the shifter column. Can someone educate me, I am assuming it is high, low and neutral for the front axle. I pulled the front and rear drive shaft out and was going to test the transfer case out in the morning. I know nothing about these machines. Any insights or suggestions would be appreciated. Thanks.