A tractor configured for grip locking during advancement to avoid slippage, particularly in open-hole wells. The tractor is equipped with a grip lock mechanism for independently locking an anchor grip in a radially outward direction. This locking occurs sequentially in advance of the power stroke of a reciprocating drive piston associated with the anchor. Thus, radial outward expansion and gripping are ensured at the time pulling of a load in an axial downhole direction is pursued. Such grip locking may be employed throughout tractoring or intermittently, depending upon well characteristics such as formation hardness and well diameter.