Thanks for the input and link anony! My son said he thought he saw that trans breather tube was attached to the firewall. In not being all that familiar with the AW4 I told him that was probably the breather for the differential while he was probably right as you just confirmed. Bet it did overheat with bubbles causing too much pressure spewing it out of the vent. Luckily it hasn't overheated since/leaked any fluid and he made it through most the steep hills in Arizona.
He called last night, the trip is still going well, and has a good friend in Phoenix he was going to try to stay with overnight. Unfortunately the guy was working the late shift so they couldn't hook up. His friend told him he has to get off route I-17 and go see Slide Rock State Park
in Sedona, AZ since it's beautiful. My son told him he's hauling a huge trailer and the guy said it's all downhill then you can jump right back on I-17 after driving through it.
So my son took his advice and drove through the park. Little did he know he would be driving down a super steep mountain side on a real windy 2 lane road with 1,000+ foot drop off's, often no guard rails, and very few places to pull off w/a trailer. Also the park was packed with people since it was a really nice weather Saturday.
My son told me he saw about the most beautiful scenery he's ever seen and drove right though the bottom of the center of the park while then getting right back on I-17. I asked him if he rode the brake going downhill and he said he just crawled down the road. Guess this still doesn't explain how he kept the heavy trailer from pushing him downhill.
He thought maybe he was about 5 hours from San Diego when I spoke with him but may stay somewhere in Phoenix overnight and take his time. Really hoping to hear from him this morning and he may already be there!
BTW, here's something funny. When trying to sleep in his XJ in NM it got down to 32 degrees overnight and he only had one blanket with him. He crawled in his back seat with his 120lb Mastiff Max and they kept each other warm overnight while sleeping in the back seat together. I'd like to see my 6'2" son sleeping with his huge dog in the not very big back seat together!