Tranny problems
 

I have a Chevy HD 2500 four door 4x4 with the 8.1 Litre. While driving to work one moring i heard a somewhat loud pop and then my truck quit pulling,after getting it towed back to the house i have let it sit for 2 months. One day while i was starting it to keep the battery charged and to run the engine, i started playing with the 2h,4h,and 4l buttons and shifting the tranny in and out. To my suprise when i changed the button from 4L to 2h and put the tranny in drive the truck moved and i drove it around the yard. I am still scared to take it back on the road do you have any suggestions on what my problem could be, also i do not hear it engage in 4L and the light will not quit blinking, so i know the 4L is not locking in. I would appreciate any help with this matter. Thanks

It sounds like you may have to inspect your differential for issues.

Tx Problem
 

To shift from 2HI or 4Hi to 4LO you must be in neutral, key on not running and foot on brake to engage. It sound like the tx case somehow shifted to neutral as you were driving. I have the same setup as you and have experienced the same problem once but have since driven 10k + miles. I would check to transfer case oil and if low it takes Auto-Trak II , not regular tranny oil.