I have not been back to this forum in a very long time, but I got an update you posted in this thread and came back to respond! This forum was always good to me and even though I no longer own a Crossfire, I can help.

To answer your questions in order:

1) There is no differential in the front, only the rear (hence why it is called a "rear wheel drive" vehicle)

2) If it sounds like a hum/drone tied to acceleration speed and only occurs in certain speed ranges (you may want to post up at what speeds you notice it), it likely is coming from the rear differential. Difficult to diagnose without hearing it. Because the rear differential is not very far behind the passenger compartment, it could be easy to confuse where the noise is coming from. If the noise is consistent at all speeds, this is likely not your issue.

3) I wouldn't call this a major *problem* but it is certainly a fairly significant repair. But depending on how handy you are, it is certainly doable. You need to swap out the rear axle entirely, preferably with a 2005+ axle since Chrysler fixed the defective clearances in the pumpkin after issues with the early model 2004s. There are tons of junked Crossfires out there - search eBay for pricing.

3.1) You can drive with this as is- likely for a long time. It's just annoying. To be safe, you might want to change your differential fluid since it is easy enough to do and will make sure your fluid is fresh.