If the oil is red-ish its tranny fluid, if its brown it is motor oil. That plug is to get to the torque converter drain plug inside the bell housing. If you pulled the plug, stuck your finger or paper towels up there and didn’t get an oil then its not your tranny oil pump seal. You can check the back of your valve covers for leaks with a small mirror. The oil would travel down the back of the motor to the spot your looking at. There really isn’t a good/easy way to check the rear main seal or the back of the oil pan gasket without either pulling the motor or pulling the transmission. They are both common leak points.
$300/400 for this job is fairly cheap for a rear main seal and/or oil pan gasket job. I’d do both while your at it.
'98 3.8L Convertible (Red w/ White top)