I tow out into sandy areas of Glamis and Gordons Well all the time with 10k bumper pull ATC. Gotten stuck plenty of times over the years. Here are a couple of go to things to consider when it gets soft or towing through sandy areas.
First if you air down the truck - always air down the trailer - it is a big drag and will bog down the truck - with truck and trailer at 25 PSI in 4L you likely would have made it through that with a little momentum.
When you feel yourself starting to bog - go for momentum - if you don't start to build any more momentum and start to "dig in" - STOP IMMEDIATLY - this is the hardest thing to do, cause you think I got to stay on it to power thru - you have to weigh - can you power thru vs being "less stuck". I have learned less stuck is better in most situation and always makes it easer to deploy your boards and less digging. Not to mention hamming the throttle and powering thru beats the crap out of the trailer and the hitch.
4L or 4H? I will take 4L every time.
if you get stuck be sure to lock the diff., then air truck and trailer down to 20 PSI - that might be enough to allow you to creep out - if not air truck down to 15 PSI (I just go there right away as you might only get one chance).
If that doesn't work, then dig - deploy boards and work your way out.
If you are in toybox - unload the SXS, quads or moto's to drop the weight if you can't get out after airing all the way down, will make a big difference to drop a 2,000 lbs. and in the case of a SXS having someone give you a small tug with that may be enough to spring you with no digging or boards.
WD on or off? I leave it on - but you can argue both sides of this - I like keeping it on so you have less load on rear wheels of truck and more weight distributed to the front. I also think if you have to "go" it takes the shocks and forces you put on the hitch and spread them over entire WD set up - so far less likely to rip your hitch off (which probably happens when they had too much load for hitch).
If you get stuck do your best to get out "without" unhooking - only unhook as a last resort when are out by yourself. Take your time and work your way out, it is just part of the adventure!
Here are a few videos on technique to crawl out of sand - this really works -
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