I'd bet my bottom dollar It's not the gears. I, like many of you had the same problem at 80 mph. Get the aluminum FMS driveshaft. It's 150 bucks and 2 hours in the drivewayto change, rather than 600 for new gears and install. You won't be sorry. 
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Proud RHC member #99
1990 Mustang GT Trick Flow heads,F303,Mac cold air,FMS 65mm tb,73mm ma,FMS 1.6,GT-40 intake,MSD igintion,NOS Big Shot@150,Mac equal shorties,Mac h-pipe,American Thunder flows,Centerforce dual friction,TTC 3550,alm drive shaft, 3.73's, HPM subframes, k-member brace, strut tower brace.. amongst others.
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