'04 CTS (problems when shifting)

Hello...
I am new to the forum and have no mechanical inclinations to speak of. I hope someone can still help.

I have an '04 CTS. 3.6L. Automatic trans. 57,000 miles. I have the lube, oil and filter done every 3,000 miles. Changed the trans. fluid at 30,000 miles.
In the past I have had problems with the battery dying and with the sunroof... but both were fixed at the dealership under warranty. Have also had a problem with the button on the gear shifter sticking. It just won't let me push the button in when it is in park sometimes. Not major problems.

I started hearing a funny sound a couple days ago.

I have noticed a change in the shifting patterns over the last couple weeks (Longer intervals between shifts, a small decrease in acceleration and a little lunge when it shifts in all gears). I am not a race car driver and most of my miles are in the city. I had taken it on a road trip and was told by one of my friends that it may have caused the computer to change my shifting points. I don't know if there is any truth to that statement... but I trust the guy.

Two days ago I started hearing a noise between shifts. It sounds like it could be grinding a little bit between gears 2-3 (might be 3-4). Maybe 45mph. I made an appointment at the dealeship immediately.
They called yesterday afternoon and told me to pick up my car... nothing wrong with the transmission. I was told that the computer didn't show any codes and that they couldn't reproduce the problem... which is funny because it's still happening when I drive.

I have done some reading and have heard that there are a lot of problems with differentials in the rear and have heard a couple stories of solenoids causing problems. Are either of them likely to cause my problems? Are there any other common problems that might cause the symptoms? Is it safe to drive?

Thanks for taking the time to read the post.

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