I found this formula after a brief google search .

power loss (in HP) = ([elevation in feet] x 0.03 x [HP at sea level]) / 1000

So, at 10,000 feet on a normally-aspirated Xterra (~170HP) you're looking at a 51HP (30%) power loss.

The effect is amplified with an automatic transmission, since the torque converter will stay unlocked in 1st gear, generating lots of heat, and if you apply more throttle, all you will succeed in doing is slipping the TC more.