Update: Pulled the trigger on an '05 X3 Thursday night. The X5 must have known something was up because the check engine light came on when we were pulling up to the dealer.
I read the code and it was a small evap recovery vac leak. (I keep the carsoft cables with me at all times. That's how "reliable" this car has been.) We just had the gas cap replaced a week before, so there must be a leaking hose in the vac system. Regardless, it was a sign. We are finished with that car.
The "new" X3 has 30K, an additional 100K warranty (including wear items like brake pads and rotors, suspension bits, etc) and free oil changes and washes for life. We got the car loaded with the same features as the X5 had, and even a few extras, so there is no real downgrade in luxury. But, the most valuable feature of all is that I don't have to worry about the car leaving my wife stranded, and if for whatever reason it does, the dealer will take care of it and give us a loaner until it's fixed.
We are happy with the choice. Thanks to everyone for sharing thoughts during this process.