My van starts to shake very badly when i go over 50 mph

With the tyres you have,being bigger than car tyres,suggest a wheel alighnment and balance.It makes a huge difference.Hope this helps

Make sure the wheels are properly balanced and secure. Check all steering components and bushings for excessive play. Make sure steering box is secure on frame. If everything is tight, you can have alignment moved slightly more positive (caster only) and that too should help.
Wheel bearing hubs can also cause shake in certain cases and a binding brake caliper can as well (one wheel will be really hot after driving when compared with others) If shake is in body and not front end (you did not say) check driveshaft joints for play.