Hi the cost listed to replace upper ball joint on a 99 Town Car per side or both

If the ball joints were failing speeding up would not resolve the issue. The 99 lincoln towncar is known for having the ball joints break causing the front end to collapse on the road. If you suspect the balls joints are worn out you should have them replaced immediately. The cost to replace upper and lower ball joint on the west coast is around $800. So both sides upper and lower would be around $1600 expensive I know! More than likely other parts in your steering and suspension are wearing out. A symptom of a failing ball joint is the car will wander left and right and require considerable intervention with the steering wheel to keep it on a straight path. It does not matter the speed and in fact the faster you go the worse it gets. In this case you will want to have a experienced mechanic inspect all ball joints and assess their condition. A lower ball joint breaking can do significant damage to your car.