If your u-joints are good,  drop the shaft and put a wrench on the pinion nut.  Has the car always done this since you've owned it? or is this a recently developed problem?  If the pinion's tight,  I'd get a set of carrier bearings and axle bearings and replace the old ones.  I wouldn't fool with the pinion bearings 'til I didn't have anything else left to check.  Before you do bearings though, you might take your driveshaft to a machine shop and have them check the balance.  Maybe a balance weight came off. 
 
 
Bring the car to Ford Nationals at Atco next weekend.  It's always easier to figure out problems by feel and listen than it is to guess at the keyboard. 
 
 
Good luck. 
		
	
		
		
		
		
		
		
			
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				Old  Slow GT
New  Slow GT--> under construction
 Latest Addition:   95 Cobra R, #73, bone stock, ~6100 miles.
			  
		
		
		
		
		
		
	
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