Sludge on skid plate

So I just bought a 1996 SR5 Xtra cab a week ago from a fellow TW member and was looking over it. It's real clean and had been garage kept. When I bought it I went underneath and looked at the drive train and everything. It all looked fine, I pointed out some bolts that had a little bit of clean oil on them and he said he's never had a drip or he would've seen it on the floor. The bolts did have crush washers So i wasn't too worried.
Today I took the skid plates off and one was caked in sludge which could just be dirt and messy oil changes. I took some before and after pics of some sludge I took off the front diff and near the shaft for the front. Also a pic of the oil pan before I wiped it off.
Just wanted to know anyone's thoughts on what to look into cleaning and maintaining next.

 

Shoulda posted in the 1st gen forum for more traffic (but here's a free bump )