Same exact thing happened to me, it was my driveshaft bolts loose, they were so loose that they were about to fall out, i posted about this, and it did the same thing you described, ask the mechanic if he removed the d/s and reinstalled it when he looked into your u-joints, i also recomend the alum d/s it was worth 169$
btw the police clutch fan failed me, just my experience with it, it may have just been old and worn out, but it came apart at a low speed and smashed through my radiator.
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Police package 5spd 90 Lx, Stock original motor, 3.27grs, BM fan, fms 10.5 clutch, D&D quadrant, and adjuster, ADS chip, 180 stat, mac cai, mac h-pipe, mac subframe connectors, ASP crank pulley, ripper shifter, 26x10.5 M/T sport pros welds. 246 hp according to analyzer 
Race weight: 3,120
E.t-13.57 with 26x10.50's e.t. streets. 1.88 60'
mph- 99.92 mph
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