Hmmm, Airlock (OP) hasn't come back in to give more precise description of the 'mud' situation he is talking about, or any other input to this thread.
Well, seeing as he said it hasn't rained for 6 weeks, it's likely he was in swampy or spring fed ground, and not climbing wet muddy ruts with wet roots and rocks.
In which case, a maybe inexperienced Jeeper (because he's asking a forum why he got stuck), A/T's in soft mud, crawling along in Lo full time is a recipe for getting stuck.
That's what happened to me for a while
Then I discovered what the hi part time can do
The full time Quadratrac-II coupler in mine works a treat. I've been playing with hi-lo tonnes over the last 2 months in the mud here, and mostly, unless the extra torque, slow climbing or descending benefit is required putting it in lo is counter productive if anything in sloppy mud on reasonably flat ground.
Plus, it's crazy going in and out of hi-lo on the one trail when you don't need to.
Sorry folks but in my honest opinion I think lo full time is over used and over rated. It's not the be all. And...full time hi is wayyy underrated.
Why lock the centre diff when the coupler works a treat in mud?
On one hand everyone says its absolutely critical to maintain four equal wheel diameters at all times, because the coupler will bind up the drivetrain if you don't...blah blah blah...but it won't work well in mud sending force to the front wheels when the rear spins
ummm.
Then on the other hand everyone says it's not good to lock the centre diff in low, on hard surfaces for tight turns...
So...a scenario, I'm on a hard pack trail, with the occasional mud hole swampy section. I'm going to stop, select lo, keep the revs up so I don't slow down too much and get stuck, go through the mud hole, stop put it in full time and negotiated some hard pack tight turns, stop put it in lo for another big hole...over and over and over. Are you mad
Yeah, forget that crap.
Leave it in full time and get more trails done
More often than not, going too slow (as noobs do in lo) in mud gets you stuck, so having to 'floor it' digging up the trail to get out is not the way to go. Keep the speed up to begin with, less damage to the trail. Starting slow and flooring it to keep the tread clear to keep moving actually has a damaging effect.
But yeah, the OP , may come back in and say it was on a steep rut infested, rock strewn climb, covered in slime that needed a careful slow negotiation...then I would put it in lo