The ornamental design for an exteriorshell of a highway tractor, as shown and described.
FIG. 1 is a right, front perspective view of an exterior shell of a highway
tractor showing our new design, it being understood that the broken lines
are for illustrative purposes only and form no part of the claimed design;
FIG. 2 is a right side elevation view thereof, the left side being a mirror
image of that shown; and