Do you have the Northstar engine or the 4.9L?
The service ride control message is one thing, a performance problem is another.
If the engine is not performing properly it can also effect the way the transmission shifts.
Let's start with some basic information.
Does the check engine light ever come on while driving?
Has the vehicle been serviced, when was it tuned up, fuel & air filters changed, etc?
If the transmission was never serviced as you say it's really amazing it's lasted this long. Even if it was serviced on a regular basis there is no garantee it would last as long as it has.
If the transmission was in fact slipping, servicing it at this mileage may not be the best idea. If you explained to the technician that you suspected a problem with the trans, at this mileage, it would be best to give the vehicle a test drive to see if the trans is operating properly. Servicing a trans with high mileage that has symptoms of slipping or other problems would not be advised. Sometimes a trans with problems will get worse or fail completely after being service. This is due to the fact that the old fluid gets a varnish or sticky, thick quality and tends to hold things together.
How is the performance now?
No, i would just keep an eye on it for symptoms of slipping more than anything else. If nothing changes in the next week or so it should be fine.
You are correct, if it is damaged the service might only speed up the process. And servicing the trans if it's healthy is the best thing you can do.
A transmission can fail at 20,000 miles or 400,000 miles. Normally when it happens is a slow progression with slipping and either sloppy or harsh shifting. Most automatic transmissions do not last this long and who knows, maybe it has been worked on before. That said, just beacaus the mileage is high is no reason to repair it if it does not display any symptoms of failing.
The rattle under the car could be the cat converter braking apart inside. You can tap on it with a rubber mallet to listen for that. That can also cause performance problems if it restrict the exaust flow.
If you are inclined, here is a link to a article on how to retreive and clear the trouble codes from your vehicle. If you get frustrated trying to do it just turn the key off and back on and start over. Once you do it a few times you will get familiar with the process.
I would try to clear the codes and see if they return.
The website also has alot of info and forums that could help you.