If a Vortec engine shakes or rumbles at low RPMs it likely needs a timing adjustment. It would also simply need a tuneup.
torque is at low rpm, horse power is at higher rpm. as you gain hp, you lose torque and vice versa.
if the steering wheel shakes there can be a problem with alignment. shaking when braking would probably be a brake problem. constant shaking i'd expect you should check the muffler If the front end of your car shakes when just driving normally, your front end probably needs re-aligning. If it shakes when you apply the brakes only, your brake rotors are probably warped and need re-surfaceing. Impossible to answer question, please be more specific. It shakes when... I feel it shake in the steering wheel when... I'm driving along and... etc. It depends on WHEN it shakes. Low speed driving: damaged tire, U-Joint, ignition misfire, burned valve, low engine RPM. Highway driving: loose steering, faulty ball joints, tire balance, bent wheel, tire damage. Engine idle: burned valve, ignition misfire, motor mounts, low engine RPM, dirty injectors, EGR valve... All of the above plus quite a few other possibilities.
Water in fuel? Have vehicle scanned to determine the problem
rpm too low maybe you are shifting to soon
tires need to be balanced or you have a bent rim or a broken belt in your tire
your car idles low because it is hard for the engine to run fast. which sounds more comfortable, 1200 RPM, or 8000 RPM? the car isn't moving, and as such it doesn't need the engine to run fast to propel it, so the engine doesn't turn fast.
A Ford Mondeo shakes at a low speed most likely due to warped rotors. When rotors are unevenly worn the surface area will not remain balanced. This then results in a shaky ride.
coil pack
Halogen fluid. When it gets low this can happen.
From my experience you get the best gas mileage when you are in the highest gear and your rpm is low, around 1500 to 2000 rpm.
My guess is its vacuum related. High vacuum at idle and low vacuum during acceleration is normal. Look for a vacuum leak