Purchased the vehicle 3 weeks ago, 257K. My 4wd had all kinds of issues. It wouldn't engage. If it did engage then it wouldn't disengage. Lots of flashing lights and beeps coming from the dash. There was definitely a problem. 

Crawled under the vehicle and noticed that a vacuum line that goes to the transfer case actuator was disconnected. The end was frayed and split. I cut off the damaged part and reattached it to the nipple on the actuator. And holy cow, now my 4wd drive works like a charm. It engages and disengages just as it should. No more warning lights or beeps. I am amazed 😮 

If anyone has this issue, start here before you pay the big bucks.