Car info pls.
If grease, faulty CV joint boot, as you initially suspected, would be the norm. If oil, hard to see how it would migrate from transaxle output shaft seal all the way to rim without leaving a trail down the drive axle.
Check tranny fluid level for sure, but fluid identification would be helpful.
Seal can be replaced by removing the drive axle, does not require internal tranny work. My guess, 2-3 hrs plus parts.
