Oh, that *is* a good question.
Again I agree the answer is "What your DM says", it's one of the fringe complications of multiclass.
Going by RAW is tricky since Wild Shape is possibly the most complicated class ability in the game and a few bits are somewhat open to interpretation. However:
You retain the benefit of any features from your class, race or other source and can use them if the new form is physically capable of doing so. is the most relevant section. - So I can't see anything in the Rules as Written that would disallow it.
Conceptually I can see it argued either way... If raging just a mental act, or is part of it the fact that a barbarian in extreme physical condition? Is this retained or lost by the Wildshape process?
As a DM the two key points to consider by allowing it or not are:
In terms of Mechanical Balance, getting the damage resistance whilst having the "Beast Wild Shape Buffer" seems incredibly powerful. So I'd be worry of the synergy of two of the tankiest class abilities in the game.
It has been mentioned that in actual play Beast Hitpoints actually drop remarkably quickly due to the amount of damage monsters do at mid-high levels vs low hitpoints.
So I would be worried it could in fact be one of the few times multiclassing does lead to a spike in power.
"Overpowered" being only really relevant if the Druidbarian overshadows another party member to the point where it impairs their enjoyment.
In terms of Narrative Awesome: TURNING INTO A REALLY ANGRY BEAR! RAAAAAAAAAAAR!
You get to be a Bear Totem BEARBARIAN! who turns into a BEAR to get the RESISTANCE OF A BEAR whilst they BEAR!
Or a WOLFBARIAN who turns into a DIRE WOLF with both pack tactics and Wolf Rage thingy and DRAGGING PEOPLE PRONE and RAAAR!
So yes, it's a difficult choice, but the latter will probably win out for me.