Our new gran prix was doing the same thing your describing. wheel vibrating, things in the cab would even start to vibrate. although the back end didn't keep loosing grip like you say. we had it re-aligned and put new tires on it and it drives like a dream!  
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1968 GMC
Original A/C, bucket seats, PS,PB
396 bored to 402, Dynomax cerama-coat headers, Super 40 Flowmasters, 272/284 Crane cam, Crane gold roller rockers, holley 750 cfm double pumper, Edelbrock Performer RPM intake, HEI ignition, 9.5:compression
Alltogether 411HP and 446ft/lbs
Dana 60 w/ 3.54 gears
Turbo 400 tranny
Why would you have anything other than a big block? HAHA
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