A few days ago I drove the San Joaquin Ridge 4WD trail. It's a terrific trail. Not too hard, not too easy. And the views are spectacular! The only negative is the length. It's only 2.5 miles long. This is now one of my favorite trails.
The trail is easier than Mengel Pass and Shuteye Peak but the views are equally stunning. I'd rate the difficulty at 2 for most of it, with sections of 3. I hit bottom once and so had to raise my suspension. The rover was gasping for air while climbing up the last steep hill, where the elevation is over 10,000 feet. I almost had to put it in low range. Here are a few pics of the trail, starting with the beginning below tree-line.
There's a tiny place at the end to make a 12-point turn-around.
The road is shared with hikers and mountain bikes. I encountered 2 groups of hikers and they kindly got off the road so I could pass (cars are not supposed to drive off the trail).
I stopped several times to take photos of the Minarets to the west and Mammoth Mountain to the south. The ground was even covered with brightly colored flowers.
From the end you're treated to an amazing 360 degree view. I found myself staring westward at the Ritter Range and the Minarets. The Long Valley Caldera is seen to the east.
To the north is Deadman Pass. Somewhere down there is the famous Lost Cement Mine, who's tale is too colorful and enticing to do it justice here. Some think it's cursed, since most who've found it, died shortly afterward. If you're interested, google it or read this short PDF.
If you're in the Mammoth area and have at least 8.5 inches of ground clearance, then I strongly recommend this trail. You won't regret the diversion from your planned activities. If you go, bring some snacks and plan on sitting for a bit and enjoying the view.
This trail is included in "Sierra Nevada Byways" by Tony Huegel (#28: San Joaquin Ridge) and in "Guide to Northern California Backroads & 4-Wheel Drive Trails" by Charles Wells (#36: Deadman Pass).
I found a couple sites that mention the trail also: here, and here.
Finding the start of the trail was a little puzzling. The sources I had were not clear. I ended up parking for a couple minutes in a dirt turnout area (where mountain bikers leave their cars) and inspecting 3 possible branches from there. That turnout/parking area is just to the right after you turn right prior to encountering the entrance booth for the road to the Devil's Postpile area (1 mile west of the ski area on 203). (Translation: drive to the turn off 203 signed for "Minaret Vista" - immediately after you turn off 203, turn into the dirt turnout area on the right side of that road to the vista) I didn't have to park actually. Only 1 of the branches was not blocked to cars. Although the only sign it had was indicating mountain bikes, the tire tracks made it clear that cars could take it.