Unimog Club UK

Smell of Ep oil when engaging the diff lock hints that the diff lock seals have gone in one or both axle(s) this means an axle split and seal replacement, not a huge job if youre mechanically minded, but does mean you have to remove the axle and it can take quite a long time to do.
try and find a steep slope (not so steep that you role it!) to climb up, engage the diff locks, if all four wheels are spinning this means they are working, if not there is something wrong with the diff locks. this can be due to something as simple as a faulty switch, but can also be quite serious.