That groaning is 'the old 2 gears at once trick eh' and there is only one thing that you can do about it. rip the gearbox out and separate the front bell housing from the sandwich plate. But try to not separate the rear case. With the selector finger still in the gate you may not be able to get the rear casing off anyway. Although it won't hurt to attempt to take the rear housing off as it would certainly make life easier. A bit of sealant after you get the thing into neutral and you are good to go. Just time.