Anything is possible. The front planetary takes more of the abuse than the rear planetary but doesn’t mean the rear can’t fail. Why it shudders in reverse? Hard to say, possibly a sealing ring on the front pump stator or lip seal in the front clutch piston. But if that were the case, similar issue in 3rd would occur. So I’m not entirely sure, just going through stuff to try and help it make sense.
With an automatic shift valve body, lots of things can get happen. I would triple check your kickdown linkage and throttle valve adjustment. If they are spot on, it may be time to pull this trans out and go through it. Especially with the possibility of a hard part starting to fail. If it were mine, I’d be pulling it out.
RJ