Question Regarding Torque - Pulling

Two motors need to be spec’d out to discover the motor-torque needed for a robot to pull with an X amount of force without the wheels slipping. (assume the contact friction of whatever is being pulled is figured into the X amount of force).
Can anybody give me any insight about how to go about this?

Good question.

I attempted an explanation here, and suggested some links for further reading.

nm figure it out