I thought I saw something on this but can't find it.

If I leave my truck in 4WD and turn off the igniton key I feel a big
clunk - thump kind of thing happening. Sort of like the driveline releasing energy or unbinding or something like that.

Is this a normal thing? Sorry for the vague clunk - thump description.

This is not going to help you but I've had that same problem before too. I haven't had it yet this year though, but I haven't driven it as much as i'd like too.

Does it do it all the time? Or only some of the time. My problem was every so often and over a year ago so i forget at what times it happend. If it is intermittent try to figure out when it does it. See if it does it after you've just turned into a tight parking spot. See if it does it after you've driven straight for a while then stopped. I'm wondering if it has anything to with the drivetrain binding on a partially dry surface and then something (not sure what) releasing when you shut off the truck.

Normal for ADD front axles.
ADD will disconnect when ignition is turned off, if there is driveline tension present it will thunk.

My 88 does this too occasionally and it only seems to happen when there is a bind on it. I am assuming it is normal at this point. I use the 4 wheel drive a lot due to the bad weather here.

Mr Pogo I like your answer. It does it everytime I turn it off. I didn't realize the ADD disengaged when the ignition went off. Makes sense now. It's probably the axles "relaxing".