I've heard that the Dirty Devil can get deeper, just as the road before it can wash out. I'd imagine if anything in the summer it should be lower. It's only necessary to cross if you want to continue on the road towards hatch canyon and the Maze district. It wasn't very deep when we went through, barely up to the sliders.
I ran it in 2wd the whole way until the crossing, although I'm sure I could have done it in 2 as well.
I'm confident that unless the road deteriorated your 4runner would make it just fine. Wouldn't be a bad idea to carry an extra 5 or 10 gallons if you plan on exploring deeper in. I think we burned about 1/4 tank on the way in and 1/4 on the way out and we turned around just after the car you can see in the above link. Not even to hatch canyon.
You'll probably want to invest in some sort of OBA though Spencer, carry some plugs, and carry a spare. Deflate your tires a bit and it'll make the ride much more comfortable.
I do have a pressure tank, 5 gallons at 120 psi, That was another thread. The concensus was that it wasn't effective enough, but maybe someone I'll go with will have one.
I have 4- 5 Gallon Jerry Cans, So that's covered I think.
I have a Spare, and a plug kit. Are the rocks sharp?
For the water crossing, I'm planning on extending my diff breather before the trip, so that should help. I know water's bad to get in the alternator, computer and Air intake, anything else?