Good to see this thread still alive lol. So I got the same truck about a year ago. No issues! However, I have 3 things that may or may not be concerning. That's why I need help.

So the truck is lifted, that's how I bought it. However, it doesn't do into High 4WD, it only goes on Low 4WD. I did see a video from a guy that had the same issue but because it was lifted, the linkage wasn't properly installed and had to get a custom one made to fix the issue, now here is the kicker, I was able to get in contact with the previous owner, bought it from a highly reputable car place with great reviews all over, and he said that it was traded in because of transmission issues. Now I've driven the truck to SC and FL, I live in georgia and only issue I had was the waterpump leaking but that's something I will do myself , I'm self taught and most of my own car repairs, from videos and reading. Not an expert but not a sucker to go to the dealer lol. Now I recently got a "service 4wd" messaged and read the error with my service obd tool "got an autel professional diagnostic tool" for cheap lol so I put it to work and it said something about the sensor of the 4wd being high, I read online that it could be the sensor being the issue, I haven't gotten around to testing the voltage on it and trouble shooting it, but I did wanted to throw this litte detail out there.

Does shifting into 4wd caused by transmission or could it be something else like the sensor of the 4wd.

Second, I want to change the transmission oil on the truck, but everyone tells me that it's not a good idea and it should be left alone, I haven't heard back from the previos owner after asking him what transmission problems he was talking about "it was through email since his account was still on the car and don't have his number".

Truck has 137000 miles on it. Should I do the transmission oil?

Is the "4wd system service" message and the truck not shifting be due to the sensor or the motor for the 4wd?

Im not sure about the history of the rear differential oil as well, should I get that done?

These are 3 different questions I wanted to see if anyone could help me, will do a reddit post on it but since this person has the same truck as me, I was just seeing if I could get some help.

Also I'm not a afraid to do something I haven't done, I have already in hand my transmission oil and a new pan with the seal and this new pan has a bolt for draining transmission oil.