Indy,

How old is your truck? Is the steering done through gears or a chain?

Either way, if you turn the gearbox so the wheel is pointed away from you, the upper plug should be just under the gear/chain to the left side. To be honest, this is tucked up in there so that it would really be easier and take less time for you to pull the motor off and fill the gearbox from the top. It is not really that big of a task, and you only need to pull it up enough to get a funnel down in there. We have done this without even disconnecting the electrical from the motor.