You mean that the oil is leaking where the gearbox and engine are meeting or it is to the back of the box? It would propably be a seal or something (check that the gearbox vent is free). Is it sure gearbox oil? If it is from a front seal it would drip from the drain that the gearbox leaves when meeting with the engine... Have you checked the drain hole for the gearbox - transferbox seal? Also front shaft seal? Reverse switch is tightened? As for the issue with the wheel speeds thats ok nothing to worry about:
rear diff is VC so both rear wheels will rotate with the same speed, middle diff is also VC so again both front and rear diff's will rotate with the same speed but front diff is not VC so a slight friction to the brake of one wheel will make this wheel to rotate slower or even stop with a result of the opposite wheel to rotate faster or with double speed if the other wheel is stopped so if you can turn them easily by hand then no problem...