Ah, the 91 560 SEL. Best of the bread
I have the same issue. Going on a year and a half since I asked about it here. Its getting worse, but slowly. I use reverse everyday to get out of the garage.
But I have learned to always park on flat or reverse grades!!! More than 4 degrees nose down and I am screwed!
So save as much wear as you can for now and avoid reverse, especially avoid revving the engine. I found that if I put it in reverse and just sit for a couple seconds it will start to engage.
I wanted to fix this last summer but could not locate the right transmission.