It's not really a "bad" noise that I can tell but, it does sound kind of like an Allison transmission when it's cold.
Only happens in 4H or 4L and goes away when in 2H.
I've checked all 3 of the front U-joints and they feel solid. I've also greased them and the drive shaft as well.

Is this noise normal or do I have cause for concern?

 

They're a little whiney in 4wd.

 

the only whine out of my 1999 with AX-15/NP231 is in reverse. Nothing that sounds like an Allison slushbox. For comparison, what year do you drive?

Change your fluid lately?

 

mine whines in 4low, pretty normal i believe

 

My 98 AW4/NP231 whines ever so slightly in 4WD. I was worried about it for a while, but it seems normal, it doesn't sound like anything is grinding, just a little whine.

 

90 RENIX and yes, I changed the fluid right after I bought it. The old fluid came out looking like used diesel oil. It had brand new fresh fluid in the tranny but, I guess nobody ever thought to change the t-case fluid..............or at least not for a good long while.