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If you've calibrated your IMU correctly then it will be producing orientation estimates in a global (truly) frame, up/down east/west north/south. If you create a static transform from your robots base_link to the orientation of this global frame. You can then produce a TF of the IMU's orientation in this new global frame. This will give you a frame in your TF tree which is the orientation of the IMU connected to the TF tree for your robot.

Once this is setup and working you can query the TF system to find the transform from the link before your end effector to the IMU frame you've just created, if you take just the orientation part of the resulting transform it will describe the relative orientation of your IMU (and therefore your end effector) in the frame of the preceding joint.

Hope this makes sense.