Also clean all earths (battery to chassis and chassis to engine/gearbox. If you haven't already done so, run a new earth from battery to engine/gearbox.
Engaging diff-lock shouldn't make it go out of high or low range.
The lever itself may be fouling on something- enough to move it forward or backward and disengage drive.
Caveat emptor.
If you go to buy a reconditioned engine you get none of the ancillaries with it.
The engine is just the sump, block, head and it's internals.
Wow, who knew my petrol motorbike with a crit air 4 sticker in the screen is actually a diesel!
Crit air is to do with emissions. Look at your V5 for co² g/km and cross reference it. If there is no figure then you will probably be crit air 4.