slight shake in old solid axle chevy?

I have three old trucks i try to maintain to no avail; these are the other two: I have an 83 Chevy 4x4 half ton truck and an 89 K5 blazer; both have 2
in lift and 33 in tires, and for some reason, around 45-50 or so, they both have a slight methodical doesn't do it if you accelerate fast enough, but if you hover in that speed it continues to do it; both of them do it; and the 83 has axles from a 78 4x4 i had and it did it as well, so it must be in an axle i think; cause the 83 didn't do it before the new (or at least new to it) axles were put under it.
Thanks again TB
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