The weird “side triangles” of GM Avalanches behind and on the sides of the rear cab window are designed to stabilize the horizontal whirlwind that forms behind the cab and at the front of the cargo box. Early Honda Ridgelines did similar.
I can't speak for GM but Honda's "sail panel" was to support the unibody that they since engineered out.
As for the OP there are plenty o aftermarket bumpers for GEN II.
