Hello,
My 91 300d did similar. I was driving as normal and at or near pulling away from a stop the gears basically dispensary. No forward, no back no anything. Thought tranny was blown so I had it towed to a friend who does tranny's from home expecting $1500-2500 for a rebuild.
He calls me a day later saying it's ready to go??? I ask what it was and I can't for the life of me remember exactly what it was, but it was some type of plastic drive gear that he had to half drop the trans to replace. It was less then $200 and out I drove!
Hopefully that is all it is and an honest shop will tell you and maybe a smarter trany guy here could figure out what part it must be.