Hey Gord, my 90 did the same thing on both back doors. I ended up spraying a ton of penetrating oil on it first then sprayed some white grease on it after to keep it lubricated. When I bought the car, bothe back doors stayed locked, and only the passenger door locked. The driver door wouldn't even lock, so for a few days it stayed unlocked. It seems to be working good, but don't use the back doors much, don't use any of the doors much right now for that matter, hehe.
SOON, I hope to put it back on the road if that damn cutlass would stop needing repairs