If it changes on decel, it's probably a changing pinion angle.
It has nothing , or very little to do with thrust angle.
With 3.91s; 130mph is 6100 rpm @ zero slip, so close to 6500 with that 727. Under acceleration and at WOT you would likely be well past max power, but with the pinion still jacked to the moon. Then when you take your foot off the gas, Compression braking will slam it in reverse direction, now pointing it down.
Under power, the pinion is winding up the diff and trying to raise the front of the car.
Under closed throttle decel, and high rpm, the pinion is raising the rear and maybe yanking the front end down.
With soft springs the range of motion could be 7 to 10 degrees. maybe more.
Your job is to either limit the range or find the sweetspot.... or both.
I went thru the same crap years ago. I added a second eye-to-eye mainleaf, and clamped the front spring sections, and briefly tried an adjustable pinion snubber; but my car was too low for it. Those changes, as well as changing the pinion angle several times, minimized the vibration, but it never completely eliminated it. Along the way I did some other things but they were inneffective. I replaced the tail bushing, had the driveshaft balanced TWICE, changed the rear yoke from 7260 to 7290. The front was already a 1350 , shifted the New U-joints, took them apart and inspected them. I ran it on the drive-on hoist with the wheels removed, the drums removed and the axles removed. I swapped to about 4 different chunks, and, 3 different sets of tires, but could never find the source of the vibration on the hoist. That's what got me started on the Pinion angle trail. I eventually took the rearend out and apart and sent the housing out to Moser's for narrowing, and straightening, along with a set of C-body axles for shortening. And you guessed it no difference.
I finally solved my problem by installing 3.55s and a 3.09 low gear, and a GVOD; and used the GV as a gear splitter. The driveshaft-rpm is waaaay slower than 4.30s, but the ratios are waaaaaay better.
Between the pinion-angle tuning and the reduced driveshaft rpm, it is now acceptable.
At 130mph my driveshaft rpm with 4.30s was ~7030, but with 3.55s is ~5800; that's a reduction to 82.5%
For you with 3.91s your driveshaft rpm at 130 is ~6390 with an effective tire rollout of 84 inches.
