are you thread locking the diff bolt?
i've never had a diff come un-done like you describe.
are they the right way round etc.?
built correctly?
i've also tried a HB One-way in the car, the driveshafts don't fit inside the outdrives, so that would be a waste of your time.
take your diffs apart, and clean everything, then rebuild as per instructions, make sure you use strong thread lock on the nut/bolt.
HTH
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