Update:
The technician examined the axles on both sides and noticed that the passenger side axle seemed to have more movement (runout?) during operation which might be causing the seal to leak. The passenger side axle was replaced with another remanufactured unit and both seals on the final drive were replaced.
About a couple hundred miles later, I inspected the final drive and there was oil droplets on the axle bolt flanges. The bottom of the final drive case had some oily residue as well but no drops.
Any ideas on what might be causing the oil seepage would be welcome.
Some pictures can be found in the diesel discussion thread here:
Final drive (differential) seepage on 1987 300TDT (S124)