1999 Cadillac Seville SLS - Electrical Problem?

I am going to take a stab at this one, and I would suspect you have a ground(ing) problem. I don't know for sure what it will affect entirely, but the fact that it shuts down when placed into gear, would lead me to check the ground strap from the frame to the alternator, for breakage. If you have(had) a front engine mount that is split, the strap takes a lot of torsion. I have seen these straps fail from such action. I can't explain why it would cause all of your symptoms, but imagine it could be possible. What, if anything, had been done prior to these symptoms? If there are any repairs that have been previously performed(related or not), take those into consideration, that the condition could have been created. The way that this vehicle's ground path is run, leaves a lot of potential. Make sure the battery cables are tight both at the battery, and where the ground cable connects to the body near the battery, and also where it attaches at the body and goes to the alternator case.