I guess the proper term would be "Tunnel brace", it is a heavy duty U shaped object that bolts to both sides of the SLP 3 point sub frame connectors tying them together for extra strength and goes under the drive shaft. I tried to find a picture of it @ the BMR site, however they do not carry it anymore.
This picture is pretty close to what I have from a different manufacturer:

This is not the BMR brace, however it will give you a good idea of what I have installed