Ok, bear with me - I am new here.

I have a 96 f150 with 4.9 I6 - 80k, 5 speed with/AC.

I recently replaced the squeaky stock belt with a new gatorback - only to have it shread off the forward facing edge two days later.

All pulleys seem to be spinning free, no strange noises.

Everything seemd to be lined up correctly and ran like a top until it ripped the edge off of the belt.

Is this just pilot error on my part, or could it be a defective belt?

 

i would suspect the drivebelt tensioner.

 

Yes, the belts wont' shred for no reason, check very carefully for a misaligned pulley on ANY pulley, at my shop we have seen all kinds of strange things do this, even cranks. Also, tensioners often separate, make sure the entire unit is not coming apart.
good luck