Rear axle or drivetrain shudder, shake, vibration under wheel spin only.

[pcweber]

I picked up a loaded 97 Expedition with the 5.4L & 175k miles a month ago. I live in Nebraska so we get snow, and have some now. Here is the scenario. In 2WD I never, ever hear this noise under normal use. If I accelerate on snow/ ice and there is some slip I get a wicked drive train vibration. If I put the switch in AWD no noise and of course additional traction so instant grip. Same for 4x4 & 4x4 low. I had a drivetrian shop tell me it is likely the xfer case chain. I guess I am trying to understand why in just 2WD the Xfer case chain would do this and ONLY when rear wheels try to spin. What I am wondering is if it is possible I have a Posi rear axle and it is shuddering somehow trying to do its thing and transferring that through the whole drivetrain? Also, does the xfer case have a mode that the chain would slip in 2WD but not AWD or 4WD? Please note this is only under rear wheel spin in 2x4, never any other time. Ideas/ thoughts?