yes testing the volt drop across the battery in the drive function is not accurate unless you put the truck in a stall or bypass mode which is not advisable. The reason being is during the scr or beginning range of the drive function the voltage is variable and not a large draw on the battery, the draw on the battery is only high when the truck goes into 1A bypass mode which then is a direct draw across the battery from the drive motor.
You can do it this way but make sure you jack the drive wheels off the floor, block up the truck too.

Since you say the hydraulics are not working i guess that way of testing volt drop is not possible unfortunatly, otherwise i would say use the hydraulics instead of the drive system.