At least the dealer is supplying a loaner vehicle.
A D99 backorder is not a real backorder. It applies only to Emergency Orders, and means that there is 'No Stock' anywhere in the system. As not all dealers are part of the system, they 'may' be able to locate a rear end somewhere (highly unlikely) and get it shipped.
The parts people really don't like to see the D99, as it means a ton of work for them, and no real info on what/when/where...so they can't even give you an ETA. They pretty much have to re-order the part every day until they get something like a D67, which will actually have information, like said ETA.
Although it really sucks, the dealer is inconvenienced by this D99 just as much, if not more, than you are, and will not receive any compensation from Dodge for their time efforts, nor for the loaner vehicle they have provided. It is in the dealers best interest to get your truck fixed and back to you, as just providing the loaner is at their expense, so you can bet they are doing everything possible to get your truck back to you.