you're really not going to be able to tell very clearly by looking at it if it's gear oil from the trans or engine oil from the engine. once either one of them starts leaking out into the open, it's going to collect all the road grime and turn black as soon. especially if it runs down the bell-housing of the trans. all the dust particles in there from the clutch and all, the oil is going to grab it all as it runs down inside
and the the "smell test" may give you a little more info on what it is. gear oil has a very distinct smell...
I don't know how often you've tasted engine oil or gear oil, but I can tell you that I've never tasted gear oil so there would be no way I'd be able to tell the what it was by tasting it, LOL. I have had engine oil splashed in my mouth before though
either one, the rear main seal or the trans input seal, if left to leak for too long will eventually get oil onto your clutch. when that happens, the clutch is shot and will need to be replaced.
buy both a trans input seal, and an engine rear main seal so you've got them both on hand and don't have to run up to the store in the middle of your work. here's the steps I'd do to go about fixing this.
1) disconnect the battery (because we'll be working around the starter)
2) unbolt the front and rear drive shaft flanges at the transfer case
3) remove the stater
4) unbolt the trans slave cylinder and let it dangle by the flex-hose it's attached to
5) go in through the cab and remove the boot at the bottom of the shifters, then remove the bolts around the plate so you can pull the shift sticks out of the trans
*you may want to try to put a plastic bag over the open hole or stuff some rags in the void to keep anything like dirt from falling into the trans
6) remove all the bolts holding the trans to the engine
7) get a jack (ideally a transmission jack) and use it to support the weight of the trans as you pull it back away from the engine
*you don't need to pull the transmission completely out from under the truck to see which one is leaking, or even to replace either seal
8) at this point, assess your situation and see which one is leaking, and replace the leaking seal
if it's the trans seal, unbolt the plate that holds the seal to the trans, pull the old seal out, pop the new one in, clean the mating surfaces on the trans and the seal housing, apply new FIPG (form in place gasket, or RTV sealant) to the seal housing, bolt the seal housing back on, and reassemble
trans with seal housing removed
seal in it's housing, unbolted from trans
if the oil appears to be leaking from the rear main engine seal, you'll have to remove the clutch, flywheel, and engine backing plate in order to get to the engine's rear main seal housing
there's 5 bolts that go through it into the block, and two more on the bottom that you pull out at the oil pan
you do NOT have to remove the oil pan to fix it. when putting the housing back on, just make sure to apply plenty of FIPG to seal up at the oil pan gasket. there's a paper gasket between the main seal housing and the block; be sure to use RTV on that paper gasket as well