This is a good question… Because if the arm is attached to the tractor, the arm will move when the tractor is lifted. I think the best way to do this is to build an armature with an two IK constraints, One in the middle and one at the end.
Try these rigs… move the control bone in the middle Bone.006 on the top rig, .007 on the lower rig, Bone.006 on the lower rig is at the tip of the bucket arm and controls the bucket arm.
Build your arm sections and use Bone Parenting to attach them to the rig. It is also possible to build convincing hydraulics with armatures or objects targeted at each other.