I took off the back wheels to take a look a the drum brakes- noticed there was black oil/grease around cylinder/ axle etc. It was dark black- How can I tell if it's coming from a leak in the rear end (is there a seal or something) or could it be from the cylinders- I'm a newbie at this so sorry if this is a stupid question- 68 SS 350 btw :D

Smell the oil on your finger. Does it smell like brake fluid or gear oil. Check master cylinder too. There is a seal on both sides of your car. You should be able to wipe off with rag and see seal around axle.

As Charlie stated, you can't miss gear lube smell.

A couple of ways to check, the leaking fluid is colored black because of the brake dust.
If axle seal is leaking, wheel cylinder will be dry, gravity thing.
Roll back the wheel cylinder cup and view the inside, if wet, leaky wheel cylinder.
If brake fluid has moistened the axle seal, figure on replacing the axle seal.