Hi mate, welcome to the forums....
So that works out at 35 to 46 mph for us UK folk
Hmmm unusual.... Just down to the low speed, usually 60 to 70 mph (98 to 112 kph) is down to wheel and balance.
So lets go through the process of elimination.... The more detail you can give us, the more accurate we can be on diagnosis and advice mate ok.
First thing I would try would be swap the front wheels with the rear wheels to see if this changes the vibrations, LR for LF & RR for RF. If that does the trick then you know its tyre/ balance or wheel only issue.
If that makes no difference, then.......
Is it a constant vibration that increases with speed and then tails off the faster you go?
CV joints/ wheel bearing/ bent or warped driveshaft issue!
Very rare for a driveshaft on a Q-system auto but the previous owner could have hit something?
Or is it a fairly fast pulsing that is constant and seems to pull the steering wheel over to either side as you turn slightly?
Power steering pump or rack issue
Check the fluid level and also check the colour of the fluid. It should be a red(cherry soda) colour or at worse light brown... If its black like old engine oil then it's time to change it.(dexron II)
Also get your whell alignment checked (tracking). It could be out if new bottom wishbones have been fitted recently.
Is it coming from just the steering wheel or can you also feel it through the pedals and floor too?
How are your brakes... Do they pulse slightly?
Do you get a lot of vibration when braking harder than normal from high speed? Discs out of true (warping). Which could also be causing your problem lower down the speed range if they are really bad... Check it in safe area to confirm... just note that high speed braking vibration is NOT the ABS.... ABS kicks in a very low speed 26kph and lower.
Try them, give as much detail as you can and let us know what happens mate... Good luck