740B Rear Axels Not Pulling Unless in Reverse

We have a 740B running on site that will not put power to the rear axels unless the truck is in reverse. We have no fault codes and the sensor to detect wheel spin on the front axel is functioning. This has my maintenance department stumped. Other than this issue the truck is in fantastic shape and runs well. Anyone seen this before? Thoughts on what to trouble shoot first?

740B Rear Axels Not Pulling Unless in Reverse