You're problem seems similar to the one I had. My truck is a 98 with the push button 4wd. When mine started acting up it would not go into 4hi, it would just blink and go back to 2hi. If you selected 4low it would put the transfer case into low range but not lock in the front axle (verified by...
I was just thinking that might be where your possible air in the system is coming from. Especially since you have gone through it and replaced a lot of components. Do you have a funnel with the radiator cap adapters? Those help alot
Thanks for the reply. Sorry to hear about your tank. I've got a similar leak on the vent and fill hose on my 98. I figured I would replace the filler neck and hoses before I condemned the tank