You are not "locked" per se at your axles.

"Locked" refers to two things turning at the same exact rate. With an open differential, each tire goes on its own path (which, if 1 gets into nasty sand/mud, it will take all the power and spin continuously, hence why a regular 4x4 will get stuck and only have 2 tires spin). This is why we used lockers, etc.

Now, when you are seeing 4low being referred to as "locked" it is referring to locking the driveshafts to spin at the same rate. Thus, 1 spin up front is 1 spin in the back. Many "fulltime" and AWD systems do not do this and that is why they are safe on pavement/traction situations, as they allow for some differentiation.

You see "part-time" because your 4WD is only to be engaged "part time" vs fulltime. The part-time system does lock your transfercase, but does not lock your axles.

4WD "LO" actually engages another set of operating gears in your transfer case. It's most easily compared to your transmission switching the rear 5 gears on a 10-speed bicycle, and the transfer case switching your front gears. In this relationship, your transfer case's "low" would be like using the smaller sprocket on the front of a bicycle vs the large one. More RPM but less stress/power to move.

Great explanation. Five stars.

From what I have read (https://www.cherokeeforum.com/f51/cherokee-comanche-stock-specs-6345/) the vacuum disconnect was only up until about 91.

Thanks for the correction, I was only trying to point out that we don't use locking hubs though. As far as I know and can find out XJ's use two locking styles from factory, vacuum actuated lock in the axle and Vacuum actuated lock in the transfercase. Some one please let me know if I'm wrong!

your jeep is too new to have a vac. disco axle. Yours is a one piece passenger side shaft. The hubs are not "automatic" they use a unit bearing that is splined into your shafts. the front drive shaft turns when you go down the road either in 4 or 2wd. the transfer case is a 231J and it doesn't work off of any vacuum. Its all mechanical. when you pull the shifter lever into 4wd high, it just moves a gear on the main shaft to lock into the front output shaft. The 231 is a part time only case. this means that it is not to be used on dry pavement in 4wd. Hence the "part time" name. the 242's were full time/part time and could be used on pavement "full time" if you wanted to. Hope this helps.

Just to avoid confusion who are you refering too? Cause I have or have had jeeps from 86-94 and am intimately aware of how my 4WD works. I personally have never owned a jeep with a vacuum lock in the transfercase but while doing some research (my 90 doesn't have a vacuum lock in the axle) I saw some write ups detailing that some have had that. BtW my 90 and 92 are both NP242's which I like cause it makes them "wife proof"... much easier to tell her to put it on the green spot if its snowing and don't worry about the other spots unless she gets stuck

so basickly if you are driving on dry pavement obviosly you want to be in 2 wheel or full time.? im kinda confused?

i'm refering to the original post. He said he has a 2000 xj. they quit using vac disco's somewhere around 1990

Yep! That sums it up right there. If you don't have a full time selection on your transfercase shifter keep her in 2 wheel on dry pavement.

And Thanks for the clarification 99NCXJ

If you're on pavement (dry/wet), 2wd is the golden rule. 4wd high is for icy/snowy roads, maybe dirt roads (if you feel so compelled). 4wd low is for the serious trail stuff.

Why use 4WDH on a dirt road? Then you can't do that cool dirt tracking around corners at hi speed!!! Remember? Like in Cars? "You gotta turn right to go left..."

Oh yeah I remember and I do that sometimes, but my 4wd takes a long time to engage (like 30 seconds sometimes), so I try and get it engaged as soon as I can before a trail.

But that's what 236k miles does to a car, everything gets a little older.

There is no vacuum disconnect on a 2000XJ unless someone put an old axle under it...and added all of the requied controls and vacuum lines to make it work. XJ's don't have Hubs in the truest sense, they have a unit bearing. The transfercase is the only means of disengaging the front axle. Since FWIW, vacuum disco's haven't been used since 92 IIRC. The front axle contains an open differential the wheel with the least amount of traction will get all of the power, not so great for rockcrawling. Your front axle is a low pinion Dana30, If you plan on doing any serious wheeling I'd suggest scouring your local junkyard for a High Pinion Dana30.

Mine's an '87 and is vacuum.