Sounds like it requires attention, is the crack running from any mounting points (bolt holes)?
bit hard to advise without some pics, but sounds like its time for some welding

Wow, that is a serious crack! If you just weld it up there is a good chance that the crack will reappear as the metal in that area will be very fatigued, if you weld it up and then weld some flat patches over the box section you may have a better chance of it lasting.