That may be it.  My 2000 S-40 developed a slight vibation at highway speed.
I thought I might have lost a balance weight, so I had the wheels
rebalanced.  That helped slightly.  A few weeks later, my son spotted a
small sidewall bulge on one tire.  It turned out there was a cut inside the
tread area, perhaps from hitting a pothole.  I'm glad it was caught before I
had a blowout.

Does it shake at a specific speed regardless of whether your coasting
or giving it gas? If yes then it's likely in the wheels
(tires/rims/disks etc.). If no then perhaps an engine mount or
transmission issue? I recently drove a vehicle (a 32' RV) that shook
like it was under attack by Klingons when accellerating in lower gears
but evened out when you released the gas or got up to a certain speed.
The trannie eventually failed catastrophically on that vehicle (black
transmission fluid!) and requires removal and $3500 work.

Good luck.
blurp