"the collar feels the same; some slack but still locked in" when the OP said this I took it to mean the Collar is actually loose but the Pawl/Locking Block is what is holding it.

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84 300D, 82 Volvo 244Gl Diesel

That's what it sounds like. He can turn the collar only a few degrees either way but it won't unscrew all the way. I think due to binding, he can't retract the pawl enough to clear the slot in the collar. That's why I said to push the pin in and lift up on the pin to apply downward pressure to push the pawl down as much as possible, and with the soapy water, hopefully will do the trick.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked