The last time I ran John Bull was 2001 I think in my old ranger. I hardly remember it. The "hardest" part of Dusy is the length. You are a long ways into the back country and if you break something it can take 2 days to get you parts. The climb up Thompson hill is tough because of its length of about a mile up a 15 to 20 percent grade. It is a mix of loose rocks from the size of baseballs up to coffee table size. If you look at it as a 50 foot long obstacle it isn't hard. It just doesn't let up for so long. This is where a doubler or a properly geared auto is critical to keep you at a controlled speed.
From Thompson Lake to Ershim Lake is the easiest section. There are a few decent obstacles but they are short. From Ershim to the end there are some decent obsticales but you have gravity working for you. My entire trip accross I think I took one spot last year. That was because there is one spot where you drive out on a rock then have to turn right at the right time. Think of it as driving a ridge at Truckhaven and knowing when to turn. The difference here it it is only 15 feet to the bottom.
Mirror has about a 200 yard section that is tough. Winch points are installed in the rocks if you need them to get out. A doubler and lockers will make short work of it.
There are about a half dozen spots on the last half of Coyote that are tough. One section that is comperable to mirror.
Strawberry lake and West lake are easy.
Brewer is easy with exception of the last 100 yards or so. It has a few decent rocks in it.
Swamp is much like Dusy. No one area is insanely hard. It just never stops for 12 miles.
I would say nothing is harder then what you would find on clawhammer in Johnson Valley. A doubler with a 5 speed is critical to making it enjoyable.
Just get your doubler installed in the Navajo. I am pretty sure you have lockers right?