I believe that crack is caused by heat stress from the weld. I would go to a frame and axle place and get a professional evaluation and estimate. Do mention the advice about using a gusset plate to strengthen the frame to the suspension member.
I have seen a lot of cracks and I am not convinced that this is one yet. I would wire wheel off the paint, and spray some brake clean on the line. If it is a crack, the last place the brake clean will evaporate is in a crack.