The ornamental design for a shockshim, as shown and described.
FIG. 1 is a top perspective view of a shock shim showing the new design;
FIG. 2 is a right side view thereof, the left side view being the same image thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a top view thereof;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a front perspective view thereof of the design in the context in which it is intended to be used, the dashed lines form no part of the claimed design.