Tractor Suspension

Im tyring to make a futuristic tractor and im having some trouble with the suspension.

I need it to work like this: http://www.youtube.com/watch?v=XaFGkuL5NBY&feature=youtu.be

and this is what i have: http://www.youtube.com/watch?v=iSm9IXVlSrA&feature=youtu.be

I'm using Edy's Vehicle Physics from the asset Store for moviment with lots of modifications and its all working well so far.

Any tips would be appreciated.