Sounds like vaporized deposits from the filaments. Must be pretty old bulbs. I'd think it's pretty rare for two bulbs on the same truck to survive long enough to develop the deposits. Usually the filaments break first.
Get some new bulbs and make sure you have clean sockets and good grounds. If the inside of the taillight housing is not clean and bright, paint inside with chrome or aluminum paint.
Some of the old glass lenses get pretty dark as they age.
Joe
