Replace, don't repair these
I'd have to agree with Gilly on replacement of the door check as the creaking sound you are most likely hearing is the internal spring of the check strap roller rubbing the ball groove because the ball under spring tension has fallen out of the check strap roller assembly (plastic end) and is no longer useable and now it's scratching the riding groove of the strap roller. If you remove the assembly with a new check strap in hand, you'll understand that replacement is the only way to go with a scratchy sounding door.
"Tell me and I will listen, Teach me and I will learn, Show me and I will accomplish, Involve me and I will succeed."
'84 300SD 256,000 Gold on Brown (Mileage Award)
'86 300E 246,000 Blue on Tan