99 TRD: Driver side rear tire w/ sticky tar substance on inside slung outwards

Hey folks, I first noticed this when I was changing out the wheels on my 99 TRD when I grabbed the tire to pull it off the hub. Starting from the axle area, and I can't see where exactly because it was pitch black out and I had a weak flashlight, there is a black (no other color contrast to it) tar / sticky / black substance slung out all over the inside of the tire outwards. There's a distinct collection of it that's accrued on the mudflap in a perfect 1 inch wide line, going all the way up and down. Dirt and what not has allowed it to cake together like a bunch of dried grease and grime. My first *hope* was that this was something I ran over, or something temporary, like a small leak that stopped a long time ago and the tar substance just never left. But after replacing my wheels, I just noticed that the new tire now looks identical to the old one, with what appears to be the same nasty black grease / grime / tar all over the inside of the tire in the same manner the old tire had it.

So that leaves me to my next fear: I clearly have spring some sort of leak. That's a shit ton of grease / fluids to have leaked out in a weeks amount of time, it will run out, and soon. And whatever part they served to protect, will be on the fast track to breaking. But the question is, what needs to be fixed? Since it's like 30 degrees and pitch black out every single day I get home from work now, it would be nice going in with a good idea of what the problem is so I'm not twiddling thumbs I can't even feel outside until I figure out the problem. Hopefully, I can use TacomaWorlds awesome vast world of Taco geniuses and go in there knowing what it is with the piece(s) that need to be fixed and just get it done.

Ahhh, wishful thinking. Anyone got any ideas? I've got a few but want to hear yours.

 

Check your differential fluid level first. Axle seal failure comes to my mind first. Also look at your shocks for a blown shock. Last, look at your brake fluid level for a brake line or wheel cylinder problem.