Could it be planted in a bad spot? I have a young plum that I'll move if it survives the year. Bad drainage in a floody season. But if your trees are 4-5 years old, it's probably too late to move them.

Sounds like a goner to me. Harvestman says they die for no apparent reason for him. They are long lived in dry climates, even cold and dry, but apparently not in humid. I believe they evolved in dry Asian climates. Cold doesn't seem to be the issue as much as humid.

My hunch is that it's toast, just wanted to see if anyone has had any experience with this happening.

Its a big tree, so no moving. It has to be root issues.

I'm on almost pure sand here...right next to the Mississippi River (If i dig down a few feet, its more or less beach sand). Drainage has never been a problem.

Maybe a combination of extreme heat, drought, long winter, deep frost and very wet/cloudy spring :)