There should be no play in the flex coupling between the trans output shaft and the propeller shaft yoke. Having said that, there is a small amount of "backlash" in the trans when in park, suprisingly about what you describe. If the flex disc is failing, you should be able to tell by holding the output flange in one hand and twisting the propeller shaft with the other. If it is worn out, you should be able to tell..

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.