Groaning noise from drivetrain AWD??

[photoexpedition]

I have a 2000xlt with an intermitent groaning coming from somewhere in the drivetrain when under a load. If i am on the road going 20mph and press the brake and give it some gas, i hear it for shure? If i do the same thing at a dead stop there is nothing so I am assuming it's not engine related. Also It will not happen when the truck is in 4wd. Only A4wd? I suspect it's something in the transfer case?? What do you guys think? Anyone else had this issue?

 

[JUST4FUN]

A4wd and 4hi are two diffrent things in the t-case
A4wd uses electric cluches to bring the front up to
speed when the ABS senses tire slip at the rear wheels
(rear wheel speed doesn't match front wheel speed)
the noise you are hearing might be the clutches engadging
4X4 hi this mode is mechanically connected with gears and
chains this is a solid connection.