2006 Chevy Tahoe 189k 3rd transmission. I am unable to find my previous post about this so just wanted to post an update. I was having the Service 4wd message and also stability control disabled sporadically. I used information on here to test the switch and indeed the 4Lo button had a bad ohm reading so I started there and replaced the switch. It seemed to help but then periodically it would come back and if I used 4hi or 4lo once I was back into 2wd it show the message every time. Next I replace the encoder motor with a no gm from the parts house. While I was doing the job I removed the ground wire/strap near the frame rail and cleaned and sanded to make sure a good connection was there as well. Once complete it seemed to work perfectly for a couple weeks then began to randomly show the message again and when I would turn the key on sometimes it would show 4lo, or 4hi or no lights or 2wd. If it was in 2wd with lights on it would shift normally but once I switched back to 2wd from 4lo or 4hi it show the message again. Anyway long story short I took it to the dealer finally. They ran the code which at the moment I dont have it to add but can later. The tech said that that code indicated either Encoder Motor error or transfer case mechanical failure. Transfer case showed no issues so we replaced the encoder motor next. As part of that a Re-flashing was suggested/required. In doing so he found a couple issues with the previous coding. There is a selection that is supposed to be A A or B B and it was set to A B so he thinks that was the root cause of the issues. SO i guess if anyone has a similar scenerio and has replaced everything I would suggest having the Re-Flash done before parts as it was only $99 and the rest was around $800 I hope this helps and thank you for all the help while working on this and input from members.