1993 Jeep Laredo Occasional severe shimmy

Suspension problem
1993 Jeep Laredo 6 cyl Four Wheel Drive Manual 140K miles

Every once in a while, at speeds over 50 mph or so, the front end will shake very badly. Sometimes it will be afetr hitting a bump, and sometimes when applying the brakes. Speed must be decreased to stop shimmy.

The following has been done but problem still exists: replaced u-joints at front wheels; inspected axles, and carrier; rotated and balanced tires; loosened then tightened upper contol arms while car was on ground.

Could there be a problem with eithe the upper or lower control arm bushings? (They were not inspected to best of my knowledge) If so, would bad bushings cause this type of problem?

At a loss. Thanks, Tim reed