The "weld" would not be the weak spot, it would be the metal right next to the weld that is weakened. DO NOT braze it, brazing needs flux, flux is acid and it is very hard to get it all out of there before painting. A brazed joint usually causes paint failures.
This is simply a vibration crack and unless you are going to drive the living hell out of the truck on poor roads and things like that, it will never happen again once it is welded.
To insure that it doesn't you could put a "backing" behind the weld made of a strip of sheet metal and weld through the crack into it.
