"The only time the vibration is present is in high gear at 90+mph and 5000+rpm. With the Ford rear I only have about 1/2 inch of play from the end of the u joint cap to where it seats in the pinion yoke. Could this cause that by the driveshaft bottoming out on the output shaft? Or would you think there is another internal issue with the transmission?"
In high gear the trans is 1 to 1. If the slip yoke was bottoming out you'd have torn up the thrust bearings inside the trans in short order.
Since the output shaft was replaced, was a new bushing installed in the front of it where it rides in the rear of the direct drum? Did you check the rear of the direct drum to see if it bent the portion that rides in the nose of the output shaft? New bushing installed in the output shaft to keep the direct drum on center? What about the tail housing and bushing? Any breakage that violent could have damaged them as well.....Cliff
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https://cliffshighperformance.com/
73 Ventura, SOLD 455, 3740lbs, 11.30's at 120mph, 1977 Pontiac Q-jet, HO intake, HEI, 10" converter, 3.42 gears, DOT's, 7.20's at 96mph and still WAY under the roll bar rule. Best ET to date 7.18 at 97MPH (1/8th mile),
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