That is a track-bar relocation bracket. It is a cheaper way to re-center the axle after a lift.
It's a piss-poor method of recentering the axle as it makes for a steeper angle on the trackbar and often induces bumpsteer. The increased lift already makes the trackbar steeper, then why add a drop bracket that makes it even steeper? It's illogical to compensate for a lift by lowering a mount on an axle, you should be raising it.
The teraflex adj. trackbar is a good piece at a good price, and eliminates the bracket.
