It could very well be that; this splined shaft is always under load from the torque converter (well when the engine is running anyway) and it is also subjected to "backwards" forces coming back through the gearbox from the driven wheels.

Every time something rough happens be it a hard shift / slamming the gearbox into reverse whilst the car is moving forward, I guess this flimsy looking splined shaft would feel it all.

Edit:-

I did add in a bit about "smacking the gearbox into park without applying the brakes and letting the car wobble 1970s springy-American-car-style" this is incorrect - all of the force would probably end up on the park pawl at the back of the gearbox. I think all of the clutches and brake bands would be disengaged so this splined shaft would not feel that effect. I'm going to check this in the manual though...

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1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!