2003 Chevy Trailblazer Front end differential

2003 Chevy Trailblazer EXT AWD, 6 cylinder, 80,000 miles

I had to replace the engine in this vehicle, which I had the dealership do. Now 6 months later (and 15,000 miles later), it was making a terrible noise from the front end. I was told by my mechanic whom I trust, (not the dealership that replaced the engine). That the front differential needed to be replaced. He told me that when he pulled the front differential off of the vehicle, that there was not one drop of oil in it. He also told me that there was absolutely no sign of any leakage of oil anywhere on the vehicle, or on the floor and asked if I had seen any sign of leakage where I park, which there was not one drop. So he suggested that when they replaced the engine, they failed to fill the front differential with oil. And that caused it to go out.I immediately contacted the dealership who did the original work. Told them of the situation. They "collaborated" with the guy that did the work. And came back with the answer that it "they didn't believe they did anything wrong. And that I would not have gotten 500 miles down the road. Let alone 15,000".I simply asked."If you filled it up with oil. Then where did the oil go?". They couldn't answer me. But still refuse to help at all with the large repair bill incurred. We are about ready to take it to small claims court. Depending on what I hear from other mechanics. Mine still insists that they could have not filled it with oil. Or even enough oil. Since there was not one drop of oil in it when he pulled it and replaced the entire front differential.

Any help would be GREATLY appreciated!