I already know I need a new sheet on the t case, and what I need to do to replace that,   but really my question is,  has anyone ever seen that sheet just disintegrate over time?   

Reason I ask is, the other day I went to autozone and had it in 4wd, soon as I went to go back to 2wd, it got stuck in neutral, and I had to tear the shifter out only to find the shifter sheet more or less just crumbled in there.   I've got no idea if this is normal of the stock sheet, or if this is some really severe issue.   

I'm positive I need to clean the material left from that out, and change the t case oil now for sure(I was due to do this awhile back anyhow) so at least I'm fairly aware of what I need to replace/change/flush.      Right now, I have no bad noises coming from the case, which isnt surprising since I'm generally pretty gentle on it. 

Yep, I've seen the stock green sheets disintegrate.  I've pulled shifters out and there are just a few crumbles left of the stock sheet.  I wouldn't really worry about the stock sheet crumbles in the tcase.....it is soft and the gears will chew the pieces up without incident.  Plus, the gear oil doesn't get pumped, like engine oil, so you don't have to worry about the pieces clogging the pump up.  But if the tcase oil needs changed, then you should go ahead and change it.

Yep, its pretty normal for the original sheets to crumble away. 

Skyhiranger pretty much nailed it.  The sheet wont really hurt your tcase and you dont NEED to change gear oil just for this.  But if you think it was needing to be changed  anyway, go ahead and change it.

well, it was on my routine maintenance list anyhow, so on that basis, it should be done.   I mean its PROBABLY only been in there since the truck was new, or whenever te last owner took it to the dealer for service, which I'm not sure of when that mght have been.