I'm in the same situation, often doing the hike solo.
Last year, I rented a SPOT. I agree w/ Steve that it's more limited than the Delorme. The cost for a one-way rental was reasonable. There are a couple of places on line where you can do this; they mail it to you, you mail it back. The SPOT does have a pre-programmed button to summon 911 help.
The cheaper, less effective solution is to rely on your cell phone. Most people's phones now get a signal at the windows, and sometimes on the summit (or elsewhere.) As you're coming down at the end of the day, you'll likely get a signal on and off on the series of switchbacks on the last stretch of the Muir Wilderness.
