[A26B]

Have you checked to know for certain what the differential gear ratio is?

Easy enough to do.
(1) raise both rear wheels off the ground,
(2) Transmission in neutral,
(3) rotate rear wheels, while watching pinion yoke, until the one of the 2 yoke joints is looking straight at you from the side,
(4) mark the tire at 12:00,
(5) rotate the wheel, counting the revolutions of the tire, until the pinion yoke has completed 1 complete revolution & no more/less.
(6) Observe the clocking of the tire mark, i.e. 9:00 for example.
(7) Multiply the clock position (example: 9:00 x 0.083 = 0.747 & add the number of complete revolutions (example: 0.747+3.0 = 3.75) and you have the differential ratio.

You don't mention the tach reading when you shift into 6th gear. What is the rpm @ 65mph when you shift?

The shake is probably due to the miss you've inquired about in your other post.