Did they make a positive traction rear differential for the 1985 Chevy pickup truck?

No, the 1985 Chevy pickup truck did not have a factory-available positive-traction rear differential. GM offered various rear axle ratios and types (like limited-slip) but not a true "positive traction" differential as a standard or option. Positive traction typically refers to a locking differential or a more sophisticated system that actively manages wheel slip. The closest options would have been limited-slip differentials, which were available as options, but these aren't "positive traction" in the strictest sense.