Ok people, this one is a MUST DO!
My truck is a 2001 and has just under 50,000 miles. My switch DID show signs of leakage. Not real bad, but it was there.
On to the information:
The new switch is part number 89059420 and has a blue inspection paint mark.
The old switch is part number 15664811 and has a green inspection paint mark. (see picture below)
You will need a 7/8" deep socket. I used a KAL tool 7/8" sender socket (see above image). It has a hex head for using a 13/16" wrench, much like some spark plug sockets. The wrench and socket worked the easiest for me.
Access with a 2" body lift is pretty good and did not require removal of the transfercase shield.
I was able to reach over the left side of the transfercase , Just above the ID tag. follow the triple vacuum hoses to the switch.
Just pull the hose connector loose from the switch.
In that photo you can see the reflection in the upper left hole from the ATF in the tube. Two out of three of them showed contamination.
Be sure to clean around the switch before removal. Use the socket to unscrew the old switch. I reused the o-ring off of the old switch since it was in good shape, and pliable. If in doubt replace it. The switches are straight threaded, and the rubber o-ring is the seal. No compound or thread sealer should be needed. Do not use teflon tape.
Start the new switch on the hole first , then slip the socket over it and tighten it. Snug is fine, don't get carried away.
Most of this is done by feel, since it is very hard to see up there. Pay attention to where the pin is on the switch. You will have to feel for it's position and reattach the hose connector correctly.
It is not too bad and only takes about 10 minutes to complete. It may take longer without a body lift since you may need to remove the transfercase shield to get enough room to work.
My price at the dealer was $29, but I talked them into $25. I wanted to make sure that it was manufactured after August 2004, that is one of the reasons that I ordered locally. They did have one of the old part number switches in stock still. Bob said that GM usually recalls inventory when there has been a revision like this. They had not heard of one for this yet though. Be careful if you are having or have had repairs done recently. You may not have received updated parts.
Here is the related document:
Document ID# 1548761