2000 Chevy Blazer LS 4x4 4.3l vortec, 157k miles.
2 different leaks.
Sorry for bad pictures.
Tell me what you can. Thanks.

 

Go underneath the truck with a few cans of brake cleaner and a pan of some sort to catch the run off. Clean off as much as you can then let the truck sit overnight, maybe run it for 10 minutes, then check for leaks. It should be easier to tell that way. You never know where the fluid could have dripped from.. or flown from.
Just realized my axle seals are leaking, meanwhile I thought it was engine oil either from the oil pan gasket or rear main just because the oil was flung everywhere then dripped down and what not. Then again, your leaks look a little worse than mine as I had an even cover of oil over almost everything..
However, the first two pictures lean towards the rear main which means you gotta disconnect the tranny. As for specifics.. someone else will have to add them as that's all I got haha
The third picture looks like a seal between the diff and the axle casing thing (can't think of the actual name)
hope that helped a bit!

 

Neither looks alarming, i would get 2 or 3 cans of brake clean and spray it all off like 01JimmySLE said, also pop that little tin cover off the bellhousing and look for oil on the back of the pan and spray in there too. A lot of times that stuff on the rear of the oil pan is from the end seals on the lower intake manifold way up top and gravity just sends it down there, not a big deal, it can also be from the rear of the pan gasket, i have a 99 with 193,000 miles on it and i just had the transmission out so i did the rear main seal but it actually was bone dry all around it, the seepage was coming from where the oil pan meets the bottom of the rear main seal adapter, to fix the right way you need to drop the front diff and oil pan and replace the pan gasket, big job....i cleaned the area and filled the area with RTV, but you still need the transmission out to do that. Also check the level of the fluid in the front diff, i would clean them up, top off fluids and drive it for a day or two and see how they look, they dont look like anything i would be too worried about unless they are leaving puddles. Not worth the effort or the money.