Out-of-round tires or rims.
I suspect some side to side wobble as well as O-R diameter.
0.125" max wheel O.R. was a number I recall, don't bet on that one though, I'm not sure. Can't recall the side to side figure at all.

Had the same exact issue with my 75 V-8 4-sp Monza, I HAD to go over the US Wide 55mph to run smooth..

A Bud had a tire "Trueing" machine, trued up the tires, No Vibration anywhere.

Drive-shaft, {Ute}? ARE WE still Talking 1200's Here ?
If so, U-Joint 'phasing' isn't as big a deal as the independent rear suspension cars unless jacked up. Still I feel the tires/wheels are the issue.
Why, cause it goes away at more speed, the "Out of Balance" drive-shafts got worse with speed.

Still make sure they're "opposed" U-know, one flops up and the other to the side?

On the 510 and the like, the rear Half-Shafts must be opposed or the car will jump up and down at walking speeds.