Im very curious why some say 4Lo because in my mind in 4Lo u couldnt do the big dunes get the speed required to climb the really big dunes. Im just curious for opinions on this choice?
Given the conflicting responses, I'm not surprised to see you (or someone) post this type of question. Like most things, there's a lot of variation based on experience, vehicles, and specific situation (e.g. all dunes are not created equal<g>). So, YMMV.
My experience is with a mostly stock '15 Rubi w/auto trans and time at Michigan's Silver Lake Dunes. If you're within driving range, they're well worth checking out.
Fairly universal, most of us agree that 12 PSI is a decent range. Need as big a footprint as you can get.
I've tried it at 4Hi, 12 PSI. Going up a ~300-500 foot 45 deg dune, I find by the time I got 2/3-3/4 of the way up, the engine just didn't have the ability to deliver adequate torque to keep going. The JK just bogs down. Part of the problem is that, with full throttle (this isn't a 2/3 throttle activity), the transmission simply wouldn't downshift quickly enough (sometimes not at all). Not downshifting when needed, you're done. Even trying to manually shift, it simply wouldn't until after I let off the throttle...see earlier result: you're done.
The other problem, stock, is that all the nanny-aides are still active in 4Hi. Plus you cannot activate lockers. I didn't make it up Test dune even once in 4Hi.
Here's the front side of Test dune:
Image is of the middle ~1/3 of Test dune. Dramatically steeper toward the left (east) end, much shallower (~30 deg) off to the right (west) end. Note it curl upward as you reach the apex. Not also (maybe not so evident in this de-res'd version) are the ripples down low.
Shifting to 4L, tires at 12PSI, went right up. Was a struggle nearing the top as a wheel(s) started to unload and break free, but made it. Subsequent runs, tried running with just the rear, and then both lockers on. Lockers seem to make it about 25% easier. In 4Lo, with both lockers on, everything works better. The trans shifts better under heavy load, you can keep momentum up easier. And, most of the nanny-aides are disabled which prevents their intruding and complicating things.
It only took one run, attempting a higher-speed run, to hit the lateral ripples, and realize I wouldn't be doing that again. Sand may squish between your toes<g>, but it can be abusively hard when it wants to. After bottoming the suspension about 3x in 4 seconds, it was clear raw speed wasn't the approach for a stock JK with 4 adults onboard.
Test dune is by far the most aggressive thing of its type that I've attempted. The rest of the dunes, far milder. Some might hit 60 deg incline, but only be a 15-25' slope.
Looking at the backside of Test dune, you'll note it's much milder, but you still have some large ripples hiding in there. Also, looking back at the ridge, the crest is just wide enough to park on, and then you're heading right back down again. Keep it moving, no tourism to be practiced there<g>.
Now, change my suspension, larger tires, and the equation could change significantly. Contemplating a lift, 35" tires, and regearing, I'm looking forward to see what changes will be noted.
. My 2014 JK was still stock then. We put her in 4Hi and disabled the ESC by holding in button for a few seconds . She is auto so you have to put her in tiptronic so u can keep rpms up and do shifting yourself. These dunes you have to do at 50mph plus to crest, had to go with a very experienced guide who knows the dunes like the back of his hand. Was one of my best and scariest offroading experiences ever. I was cured of steep inclines and downhills from there on. Haha