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Check and make sure you don't have any bubbles in your tires, but that should be even more noticeable at low speed.
I also just had to swap my driveshaft out for a one piece after my lift. My carrier bearing bushing on the driveshaft got eaten up and it caused a similar vibration, but it got way worse after a week or so.
Make sure your tires are balanced.
Also, I've had a few vehicles that would shimmy/shake at speeds like that because of bad ball joints. This is especially noticeable if you have a little "slide" feel when you turn in one or both directions, like the truck will kind of shift one direction or the other very slightly. This could also be true for practically any rubber or similar suspension component that could be worn out and create slack in the system.
I have a 93 ranger 3.0 and it starts shaking around 35-40. It some times feels like its bucking.
I just changed plugs & wires, and also the wheel bearings and the shocks and still no luck.