Dasfaust,
I took another look at your photos. I think someone has not connected your lift arms correctly. The front pivot points should be under the rear axle, not on the framework that surrounds the PTO. If you look at the photo I posted the front arm pivots are on brackets that share mounting bolts with the fenders. It looks like your setup places the implement about 6" back from where it should be. No wonder the front end comes off the ground. In addition the lifting arms (I'm running out of terms) should be attached near the rear of the lift arms. It looks like yours are attached to the center bolt holes where the turnbuckle attaches. You must have a strong hydraulic system to be able to lift the brush cutter, because you are at a great mechanical disadvantage.
Jim