This harmonic balancer repair sleeve might be useful. If you have cleaned off the oil and grime, you should be able to tell an oil pan leak from a rear seal leak. The rear seal leak is almost a 100% possibility. An oil pan leak is a lower possibility but not uncommon.
There is an old adage that goes
"if the old truck is not dripping a little lubricant, it needs lubricant".
Regarding the road draft tube being plugged. This is easily removed and checked/cleaned.
Tim
