Eeeek! Chassis flex K10 problem. Help!

[triggerthetruck]

Help please with information...has anyone experienced chassis flex on the front of a K10? This is...I have just fitted new leaf springs and shocks...and have fixed a lot of my problems which were to do with rough ride...but another problem still remains which is that from time to time, when I hit a pothole/bump...the chassis can ossilate/flex very rapidly...very scary...and the only way to stop it seems to be hard braking...which is sometimes tricky for any following vehicle. Scares the crap out of me too. Has anyone had this and fixed it? I imagine that boxing the chassis rails would sort it...or welding up the rivetted cross members...or adding another cross member? This is really quite a serious/dangerous problem, so if anyone can help me out I would be very grateful. Andrew