Hello RyanB, thank for your reply, I already try the "Keyframe Reduction" to "Off" with no result ... ( for the rig option I'm in legacy mode / Generation: store in root) For the second solution, How do you set up IK handles in Unity?
Try selecting your FBX you imported into Unity. Click on the Animations button in the inspector. Switch the Anim. Compression from "Keyframe Reduction" to "Off". That may fix the problem. P.S. - I would set this up with IK handles in Unity. It will make the arms move naturally while still holding onto the cubes.