If it's oil, it's coming from the rear main seal.. I would go with this idea.
The seal doesn't cost much but the labor to get to it does.
To give you an idea what is involved the procedure is illustrated below from the service manual:
Removal & Installation
- Drain the engine coolant.
- Drain the engine oil.
- Remove the oil pan.
- Remove the engine flywheel.
- Remove the rear oil seal housing and gasket.
Rear seal removal 3.8L
- Remove the rear main seal by placing the housing face down on 2 blocks of wood. Using a drift tool and hammer tap the seal out of the housing.
Do not lubricate the rear main oil seal or the installation tool. Do not touch the sealing lip of the rear main seal once the protective sleeve is removed.
Use a clean lint free cloth to clean the crankshaft sealing surface. Remove sharp edges or burrs that could damage the rear main oil seal. Remove burrs or sharp edges with a crocus cloth.
- Carefully remove the rear oil seal from the protective sleeve.
- Install the oil seal onto the seal driver EN-47623 or equivalent by placing the seal at an angle and twisting until it is fully seated.
- Install the seal into the housing using the seal driver EN-47623 or equivalent and driver handle J-8092 or equivalent. Apply downward force until the seal is seated.
NOTE: Ensure that the housing gasket is installed correctly and that the oil passages are not blocked.
- Using a new gasket Install the rear oil seal housing on the engine being careful not damage the rear main seal.
NOTE: The plastic inserts are used in production and are not needed for assembly.
- Hand tighten the bolts.
Rear seal housing alignment 3.8L
- Place a straight edge on the crankshaft rear oil seal housing flange and the engine block oil pan flange. Using a feeler gauge check that there is no more than .0004 in (0.10 mm) step on each side. Rotate the housing to make the steps equal on both sides.
- Tighten the housing bolts 11 ft-lb (15 Nm) plus an additional 50 degrees.
- Ensure that the housing did not move out of specification. Repeat step 6 if the oil seal housing is not within the specified range. Replace the rear oil seal housing if the specified measurement can not be reached.
- Install the flywheel.
- Install the oil pan.
- Fill the engine oil and the cooling system.
- Check for leaks
- Remove the flywheel from the crankshaft.
- Remove and discard the crankshaft rear oil seal from the rear cover.
- Use clean engine oil to lubricate the outside diameter of the crankshaft rear oil seal and rear cover oil seal bore.
- Install the tapered cone and bolts of a J 41479 or equivalent crankshaft rear oil seal installer onto the crankshaft.
- Place the crankshaft rear oil seal onto the tapered cone.
- Install the threaded rod of the J 41479 crankshaft rear oil seal installer into the tapered cone.
Rear main seal installer 5.3L
- Install the crankshaft rear oil seal by turning the handle of the J 41479 clockwise.
- Remove the J 41479 crankshaft rear oil seal installer from the crankshaft.
- Install the flywheel onto the crankshaft.
I'm not sure what the exact cost would be as the hours were not listed for this procedure on my online resource.