Weird groan leaving a stop.

[BR3W CITY]

My 05 Yukon has a weird little issue, that I'm not even sure is an "issue" as much as I just don't know what its doing.

When leaving from a stop or low speed accel, I get a "groan" until I get to around 9-15mph. Its not everytime, and it doesn't seem to react to particular conditions.

It sounds a bit like a power steering groan, but it feels like its coming from a bit further "back" than a PS issue usually would, and it doesn't react to steering input from what I can tell.

Any thoughts? Possibly something in the transfer case or 4wd system?
Its snowing AF out today so I'm planning on playing with the 4wd a bit to see if being in Auto, 4l or 4H makes a difference.