More details please, how are you trying to engage 4Lo and are you barely rolling, stopped, or driving while trying to shift into 4Lo? Do you have a manual or transmission?
Have you ever had it in 4lo? It takes a firm pull, a bit to the side first and then pull it back per the diagram on the handle. The shift mechanism might also just slipped out of adjustment which happens. That typically allows it to shift into 4hi but not 4lo.
By loosening the nut you can move the actuator on the transfer case independent of the lever.
on the number 241 Rubicon transfer case you move the actuator toward the back but I don't know on your particular model.
basically I like to move the actuator all the way into 4 low with the lever up high and then reset the bolt tight ... Then you have 4 low as a baseline
there is a good video online of a guy doing that he's got a green Jeep up on a rack and he walks you through the process.
it's really not a mystery you just need the lever to be in the correct position relative to the position the actuator is in and then tighten the throw rod.
just know if the jeep has been lifted it alters the relationship of the linkage to its own components and typically you just don't get proper throw and normally you end up with other linkage issues such as the linkage prematurely failing.
Don't let up till you get it right... You don't want to be out in the woods with it jumping out of 4 low .
The Jeep virtually has to be at a dead standstill to go in 4 low maybe just creeping in order to get the gears to meet up because I don't believe they are synchronized.
Don't be intimidated the transfer cases are extremely tough.
The stock linkage system concept probably predates our births...It's inefficient and it's a very complex means to an end... The cable system does away with the vague feel that you get..to give you a comparison the later models have a cable system from the factory.
Good luck stay on here if you own a TJ ...it will help you.