I took it in for an oil change last week and they let me know something was leaking - in the general area of the rear main, the hydraulic sway bar disconnect and the front diff. All had indications of recent oil accumulation. I sprayed 2 cans of Gunk engine degreaser to clean it off and then drove it a bit. Fresh oil was on the bottom of the front diff but I really couldn't say for sure that was the source. Today I put it up on the lift and removed the skid plates for a better look. It then became obvious it was coming from the pinion seal on the front diff. I'm hoping my extended warranty from Granger covers this!
I guess from those 3 possibilities, the pinion seal is the least expensive and easiest fix. It took most of a quart of gear oil to fill up the diff.