We claim the ornamental design for a truck rack base member, as shown and described.
FIG. 1 is a perspective view of a truck rack base member;
FIG. 2 is a back elevational view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a top plan view thereof; and,
FIG. 7 is a bottom plan view thereof.
The broken lines in the drawings are used to show environment and unclaimed features of the truck rack base member. The broken lines form no part of the claimed design.