stuck in 3

[Sid The Kid]

Hi
I have a 1991 500SL. MY transmission will not shift out of third gear. The fluid looks clean and is not low.
On the highway it will just stay in 3rd gear and if you accelerate hard it just jumps right to the red line without changing to fourth. This makes the whole car shake because of teh rev limiter. Sometimes it will go into fourth, but lately it hasnt been going into 4th too often. does it need a filter change or is there a problem?

[C32AMG]

There is a problem with the 3- 4 shift valves in the valve body