Rear-end venting/blow-by when towing???

So I've read a lot of threads in here about the diff/axle seals leaking when heating up, but not sure that's my problem. My truck is the '05 with the finned diff cover, wheels are clean and no evidence of seals leaking. Actually, the only thing that's really dirty/soiled under the rear end is the black box that sits directly behind the fuel tank. When I noticed it was towing my 14' Starcraft pop-up (decent weight, but no where near limits) from Lake Havasu to Williams, AZ along I-40. That trip is all up hill for about 120 miles. I tow carefully and within all guidelines. When we arrived at campground, I noticed some splattering of an oily substance on the trailer tongue. I assumed it came from under my truck and that's when I noticed the black box being grimey. Any ideas??? What is that black box behind the fuel tank? BTW, Nissan dealer said nothing wrong when changing out the fuel unit recall.