The rear tires on my '87 GT are wearing excessively fast on the outside. We just rotated them about a month ago and already most of the tread on the outer edge of the tire is worn.

The car sits at a stance where the top of the rear tires is out an inch more than the lower part of the tires. What could be causing this?

Get wheel alignment.

has car been lowered?
Lower cars often have alignment issues.

------------------
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)

quote Originally posted by bishop144: The car sits at a stance where the top of the rear tires is out an inch more than the lower part of the tires.

Bingo! When viewed from the rear, the tires should tilt inwards at the top by about 13/32" which equates to 1 degree if you're using a 25" tall tire. In contrast, you've got a 2.3 degree outward tilt which is a full 3.3 degrees away from where it should be. It may not sound like much but that's what's eating your tires.