Definitely the pinion seal. If it's not spewing, make sure the fluid level is good and you could get by. If there's a lot of fresh, dripping oil- yeah, it need to be replaced.
It's not hard to do in the driveway, you just need a good diagram of the differential/ring&pinion and the right torque settings. Basically, you're unbolting the rear driveshaft from the rear axle pinion yoke, removing the yoke, prying out the pinion seal, installing a new one and putting eveything back together again. It's a lot easier to when the pinion is out- but it doesn't come out that end of the axle housing!