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~some~ rotational 'slack' in the driveshaft is normal... up & down or side to side is NOT normal. Fords are notorious for loose gear lash; nothng wrong with it, just the way they were engineered. Unless you're hearing some darn awful noises comming from back there, I wouldn't lose much sleep over it. If you don't know the last time the differential oil was changed, now would probably be a good time for some fresh 80W-90 
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1965 GT coupe, 333ci aluminum headed/solid cammed stroker, four speed, 3.70:1 9"
All Blue Oval, no blue bottle
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[This message has been edited by V8 Thumper (edited 12-22-2003).]