Funny, I was just in Death Valley 2 weeks ago. I carried nearly all the tools that have been mentioned so far, including serp belt, u-joints, rad hoses, etc. I thought I had everything covered that could possibly break, but in the end, the only thing that got us out was the satellite phone I rented. HIGHLY RECOMMEND.
We had stopped for lunch on the Echo Canyon trail off Hwy 190 and when we went to start up the Jeep (2003 Rubicon), we got nothing. It was weird. All the electrical worked but when I turned the key, nothing. I inspected all the wiring, tried to bridge the starter, checked all the fuses and relays, and finally beat it with the BFH. Nothing. We were by ourselves on a 110* day but had the food, water, and shelter to keep us safe. Totally dead in the water, completely out of options, I used the sat phone to call a tow truck out of Pahrump, NV the next day.
Turned out to be a faulty ignition switch, something I couldn't have fixed on the trail anyway. Point is: be prepared, but I think a communication device is the most important thing you can bring.
That and a credit card with a high limit, it was a PRICY tow and fix!