Just purchased a used 1998 S10 extended cab 4 cylinder automatic had a slight vibration around 60 to 65 mph, I took it into the mechanic and they had all of the U Joints and the carrier bearing replaced also had the tires balanced and rotated. Now the vibration has turned into a really bad shake but the mechanic said he cannot duplicate the problem. I can't drive the truck above 50 mph.

Could be Is run to rich. You can have it tested on a machine at your local garage. Or you can try using Gas treatment or fuel injection cleaner to see if that helps. ever witch one you have.

test the front end completely. You must go to the mechanic, the tires specifications you must see

God bless you

recheck balance and inspect threads can have a possible bad thread belt on tire

VIBRATION - DRIVELINE RESONANCE/VIBRATIONBETWEEN 1400 AND 2000 RPM WITH CONVERTER
Clutch ENGAGED DURING MEDIUM TO HEAVY
THROTTLE CONDITIONS - VEHICLES WITH 7.3L DI
TURBO ENGINE AND TRANSMISSION BUILT PRIOR
TO 11/13/95 (5L13)
^ TRANSMISSION - E40D - DRIVELINERESONANCE/VIBRATION BETWEEN 1400 AND 2000
RPM WITH CONVERTER
Clutch ENGAGED
DURING MEDIUM TO HEAVY THROTTLE
CONDITIONS - VEHICLES WITH 7.3L DI TURBO
ENGINE AND TRANSMISSION BUILT PRIOR TO
11/13/95 (5L13)

FORD:
1995-1996 E-350, F-250 CREW CAB, F-350 CREW CAB
Article 96-26-21 is being republished in its entirety as part of a consolidated 7.3L diagnostic aid booklet.
ISSUE
Some vehicles with transmissions built prior to 11/13/95 (5L13) may exhibit a driveline resonance/vibration between 1400 and 2000 RPM with the torque convertor engaged. This may be caused by the torque converter.
ACTION
Prior to torque converter replacement, refer to the following Service Procedure for details to verify conditions.
SERVICE PROCEDURE
1. Verify the resonance/vibration does not occur in neutral and is most noticeable between 1400 and 2000 RPM with the converter clutch engaged during medium to heavy throttle conditions. Also watch for the vibration to diminish at engine speeds lower than 1400 and higher than 2000 RPM. If the concern meets all of these conditions and the transmission has a built date prior to 11/13/95 (5L13), replace the torque converter. Refer to the appropriate Service Manual for removal and installation procedures.
2. If the vibration concerns do not meet the conditions, check for non-factory installed items that could affect driveline and verify proper installation. Also refer to the appropriate Service Manual, Section 05-00 for diagnosis and testing.

Probably a Bent axel I had had same problem

your best bet is to get a harness from the parts store for your truck. cost is around $30.00 but well worth it. it connects into your harness and doesnt corrode and blow fuses while your trailer is in the water

check u joints, also could be flywheel or torqe converter

the first thing i would check is to see if 1 or more of the tires has a bubble.

The RH (Passenger's) side Cylinders 1, 2, 3, and 4 The LH (Driver's side) Cylinders 5, 6, 7 and 8

since you are getting your car aligned make shure that they check the constant velocity joints.....

when these are bad...what you get is a vibration in one o both sides, also you might hear a tapping sound when steering.