What is the difference on a 1991 dodge diesel pickup that has sticker back of bed says beast from same year model truck does not have sticker?

The only difference between a 1991 Dodge diesel pickup with a "Beast" sticker on the back and one without is the presence of that sticker. The sticker doesn't indicate any factory-installed performance modifications or different mechanical specifications. It's purely a cosmetic aftermarket addition, likely applied by a previous owner to express a personal preference or perceived image.