G'day,
Was underneath the rear of the car the other day and noticed the rear diff had been seeping oil out bottom half was covered in a thin film. It was also covered in dirt (as I have recently been to the Inlaws farm). Should I clean it with a can of degreaser and see where it is leaking from? Next service is due in 20000km (as it was done 20000k ago with the transmission and I haven't been looking underneath at the rear since the last service mainly front end with brakes and engine etc). the only other rear diff I have ever dealt with was on my Gemini 17 years ago, it did the same thing, but I put that down to it being a holden.
I'm thinking they didn't replace the washer on the drain or refill plug, or a seal is going?

Thanks in advance.

I would give it a good clean down and keep an eye on it, may be the front seal but would be the first time I have seen one leak to be honest

Sounds like the seal to me. Same thing happened to my car on the front diff - a small amount of oil seeped through the seal. Very inexpensive and easy to fix.