I go down a steep gravel road that at one point is 1/2 mile concrete ramp. I use 1st gear to try to reduce wear on my brakes - how many RPMs are safe? Is there some speed at which syncros or other transmission issues arise - I am not getting anywhere near the engine redline RPMs, but I know that is probably set for circumstances other than gravity pulling me to a high speed/RPM.
Is your transmission manual or automatic?
What is the model year of your Tribeca?
What is the redline on your tachometer?
I have the same 3.6 liter engine in my 2011 Outback, and it is redlined at 6,600 RPMs.
That being said, I would not be comfortable in this situation if the engine was turning at more than 4k for 1/2 mile or more, even if the engine can safely rev higher than that. If I was in your situation, I would use both the brakes and the transmission to control both vehicle speed and engine speed.
Don’t worry about your brakes. They cost a fraction compared to repairing your automatic transmission. You also waste lots of fuel revving excessively.
I would not allow the motor to rev past 3500 rpm. So use first or 2nd gear as appropriate.
Brake pads are the cheapest part on your car. Use them.
“You also waste lots of fuel revving excessively”
I doubt the engine is using any more fuel than at idle, as the computer would not inject any additional fuel seeing no throttle angle.