There is a dark area covering the bottom 3rd of my differential gear housing. I'm thinking its an oil leak and am wondering what I should do with it. Should I take it to the nearest shop? or should I jack it up, remove the casing, drain it, and replace gaskets etc.

 

Depends on where it's leaking from. If it's the casing joint, you need to drain it, pull the axles partially out, unbolt the diff, pull it, clean the mating surfaces and use silicone as a sealer, then reassemble. Do not unbolt the pinion nut unless the pinion seal is leaking. If that's leaking, take it to a shop that knows how to set backlash of diffs and let them do the work.

 

Before you do anything make sure the drain plug or fill plug is not leaking.
I would thouroughly clean the surface off and drive it some to pin point the cause of the leak.
1st check to make sure the level is full.
If it is not the drain plug or fill plug that is leaking and it is the actual casing to housing that is leaking. You can either reseal (They don't have a replacement gasket) they use gasket maker from the factory.
Or you can try the external aerosol sealant where you do not have to disassemble and see if that works.