Oil leaking between my rim and the final drive housing

[jtascone]

Hello all. I had my swingarm and final drive/u-joint service done about 4000 miles ago. My bike just turned 20,000 miles. It's an 08 Nomad 1600. I noticed while checking my tire pressure today that the left side of my rear rim is coated in oil and road grime and I have little black lines that spider from the rim onto my whitewalls. Upon inspecting everything, it appears that the final drive oil is leaking between the housing and the rim. The front and rear of the driveshaft all look good. It's only leaking around the housing. I replaced the final drive oil about 3000 miles ago and did so as per everyone's recommendations, leaving the bike on the sidestand so as to not overfill it as I have read that sometimes can cause a leak. Can anyone tell me what could be causing it to leak out around the rim where it meets the housing and what's entailed in fixing it? Thanks. By the way, there is no oil dripping on the ground so it must just be leaking only when the bike is being ridden.