Can you be more descriptive of the vibration. Do you feel it in the
steering wheel? How about when you are braking? Even new tires can be
bad (broken belt).

Did you have them check the tie rod ends and idler arm? Both can be
problem areas for a 1997 Expedition.

Have you tried rotating the tires (front tires to the rear, rear tires
to the front).

Are you running the proper air pressure (OE tires were likely to be 35
psi, have no idea what the correct pressure is for the size you are
using now, probably 35 would be OK).

You probably should have the runout checked on the wheels. Some alloys
are bad from the factory - particularly if they weren't designed for
that vehicle. The 1997 Expeditions were intended to have hub-centric
wheels. This means the wheels are supposed to be centered by the
raised machined boss at the center of the hubs and not by the lug
nuts. Aftermarket wheels are not necessarily designed for this type of
mounting (or require the addition of centering rings). If you are
using the wrong type of wheels, it is possible they are not properly
centered on the hubs.

Is the vehicle 2WD or 4WD? Some 2WD 1997 Expeditions had problems with
vibrations from the driveshaft (see Ford TSB 99-15-7 JUL 99
Drivetrain - Vibration/Shimmy Above 45 MPH). It seems unlikely that
this problem would not have been corrected long ago, so I doubt if it
is your concern.

I suggest you join the Expedition Owner's Mailing List, and pose your
question to that group. See
http://mail.xpog.com/mailman/listinfo/expedition

Ed