The ornamental design for a drop down trailerhitch, as shown and described.
FIG. 1 is a front and right side perspective view of a fixed drop down trailer hitch showing my new design;
FIG. 2 is a right side elevational view thereof;
FIG. 3 is a left side elevational view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a bottom plan view thereof;
FIG. 6 is a rear elevational view thereof;
FIG. 7 is a front elevational view thereof; and,
FIG. 8 is a front and right side perspective view of the fixed drop down trailer hitch with related environment in broken lines. The broken lines form no part of the claim and are shown only as relatable environment.