Q: I had no oil in the differential and now there is a loud knocking sound from the transmission. Difficult to go into gears. What ne

If there was no or little oil, that means you had, and still have, a leak probably through a seal or gasket- although sometimes differential or transmission cases can crack and leak that way, too. Knocking sounds of course signify a mechanical fault or worn or broken part. The origin of the sound can be pinpointed to the "major assembly", that is transmission versus differential, or sometimes both, that is faulty but then those units will likely have to be disassembled to replace or rebuild the faulty components. That is a fairly significant undertaking and YourMechanic can certainly give you an indication of the cost and get you linked up with a Specialist. Of course, before all, a diagnosis has be to be performed...you want to make sure that whatever is removed is actually faulty.