Wow! Worn splines happens once in a blue moon, particularly if nobody ever lubed the moon.

I assume you're chasing a vibration problem.

I'd grab the drive shaft in a good car, give it a wiggle, and judge the amount of play in the splines of a non-vibrating drive shaft. See if it's perceptibly less than your "worn" shaft.

The cars having A/T with OD (I think '82 might have been the first year) will develop a drive shaft vibration as the tranny output bushing wears. Usually they also leak tranny fluid from the worn seal. I've done five to six of these, two in my own cars. I do not know if the older A/T suffered the same failure or if 4-spd/OD trannies ever failed this way (none of mine ever have, even with over 400k miles).

When you install the drive shaft, be sure to maintain the alignment of the two halves. Be sure to generously lube the splines with good chassis grease. And be sure to refit the rubber condom-like thingy seal at the splines.
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Don Foster (near Cape Cod, MA)