I noticed this on my center axle before it meets the rear differential when i was under the car today. it smells like diff fluid for sure.
Does anybody know what is leaking and what i need to replace? and is it hard to replace?

 

The wet spot in only one place on the companion flange says no but the lube line on the body and exhaust where oil has been flung says yes. Weird that it would leak from only one spot on the shaft. Maybe there's a nick in the shaft where the seal runs.

 

well thats a good observation. i didnt notice the residue on the body.
what would i need to replace exactly? and is it as simple as taking the shaft off?

 

Well I've never had an Evo rear diff apart and I hate giving advice concerning something I haven't done.

Per the service manual the torque on the nut in the center of the companion flange is 137 ft lbs. So, that would be a tussle. Second, the drawing depicts using a puller to get the companion flange off. Finally a seal driver is shown. So, it looks doable but not easy.

As for what to replace, I'd check for any play in the pinion shaft (this before you remove the nut). If play then new bearings. I'd check the companion flange, where the seal runs, for nicks or grooving. If damaged, new companion flange. Finally, I'd install a new seal with a thin coating of RTV on it's O.D.