.....and no, I did not just consume the burrito blue-plate lunch special:

Yesterday we took a 150-mile drive around the Belgrade Lakes in central Maine. Lots of hills, some off-road, but mostly easy on-road stuff. 75 degrees and dry.
After dark we decided to take the interstate home - a 50-mile trip. After 20 minutes or so there was a strong smell like hot tar or hot/burning rubber. It was very noticeable coming out of the heater, but after turning it off we could still smell it.
We were doing +/- 60 mph with the hubs unlocked and the o/d in. My engine is new (3k mi). I did notice after getting home that my o/d and transfer case were fairly hot - normal, I assume? Next day I checked my fluids - all full. Nothing appeared to be burned, and all electricals were working fine.
Any ideas? I am assuming that the (relatively) high speed run had something to do with it, but I've done this before with no accompanying stench.
I await sage advice from the group.