When in low range, the pump flow goes through the front transaxle motor and then on to the rear motor, which drives the mechanical rear axle.
So two items to look at: First is that it is in low gear (there is no assist in high gear) and second would be the 4wd valve is being energized when in low range. The valve is located mid ship on theright side (has a little red button on it). It could be electrical.
There is an electrical schematic in the Owners Manual or Parts Book. You should be able to trace out the circuit using it.
The Toro Company