Rear differential vent tube?

Might be grease from the rear U-Joint when greasing if it has a grease-able fitting. If the rear end is dry I doubt that it would come from there unless the pinion seal is leaking where the drive shaft connects to the rear end. Make sure the grease level in the rear end is at its proper fluid level, you don't want to run with that empty.

The oil smells like gear oil from the rear end.i just can't figure out where it is coming from though

I mentioned where to look, see if you can see if there are any wet looking areas on the rear end of the vehicle, especially the the area where the drive shaft connects to. May be a leaking pinion seal.

Axel seals and diff cover are the only things that are leaking, I can't figure out how the oil is getting all the way up in the bumper

Axle seals leaking are a good indication of a possible bad rear axle bearing, but not always. The rotors may be flinging the grease up to the bumper or if it drips out of the rear end cover, just the air flow while traveling will fly up to the bumper. Never the less it needs to be checked out. You do not want to run without any grease in the rear end.

How do I check if it has the limited slip or not so I k ow if I need to put the friction modifier in it or not

If you go to your local parts store, they will look it up and tell you what you need. Just ask them, they will give you the right stuff.

It doesn't look like the seals are leaking anymore, I have no record of them being changed in the last 8 years when the old owner was notified, I've never put fluid in since I've owned in the last 6 months, but I took the fill plug out and the fluid came running out

Then the fluid is full. Hard to say what it is exactly the stains are from, but it doesn't sound like anything to worry about.