2005 GMC Envoy Wont rev above 2000 RPM while in gear.

My son called today to tell me about an issue he's having with a 2000 GMC Envoy. He's in college and about an hour drive away so I'm having to troubleshoot remotely.

The car seems to run fine other than it will not go above 2000 rpm when in gear. In neutral it will rev above that and it seems to idle fine. It's not rough or surging.

When trying to drive the car it's slow to accelerate because of the issue but will eventually get up to traffic speed.

I had him check fluids and the trans was about 2 quarts low. I had him top that off and drive the car around the sparkling lot. He said it didn't seem to make a difference. The low fluid may just be coincidental.

What else could be causing this issue?

If the "Service Engine" message is not present, it may be insufficient fuel pressure, restricted exhaust, or some other system which isn't directly monitored.

I understand that you are probably working with limited information and input, but so are we.