[NAJ]

I assume you are refering to the seat belt retractor assemblies? Is the problem that they will not retract or they will not lock on sudden stops. Failure to retract is a common problem even in todays vehicles, you can remove them and try to clean and lube the assembly but I have never been able to correct that problem other than replacement.
My driver seat belt will not retract, I have to move it in by hand usually after removal, not putting it on, it stays tight and does lock so I am fine with that, it is not a saftey issue as long as you can make it tight and it does lock. Mine will lock me in the seat on a hard shift.

 

[NAJ]

Your problem may be that new are probably no longer available.
I bought a new Mopar for my D/S about 7 years ago(too lazy to put it in) and it is now discontinued.

 

[NAJ]

I do not think so, they react from change of momentum.
Yours is the first one I heard of that will not lock, usually they won't retract so the odds of finding another that does not lock is probably slim.