This problem sounds like the first and last time I had my first car serviced by any professional mechanic.
When they rebuilt your gear box they might have over torqued the adjustment screw in order to compensate for the loose front end.
Worn tires won't help in this situation either.
Get someone to turn the steering wheel back and forth, 10 o'clock then 2 o'clock positions, while you look at the steering linkages. EVERYTHING must move smoothly without wiggling or have any noticeable play.
To check the ball joints, lift each wheel off the ground, by jacking it up by the knuckle not the frame, just enough to get a big crowbar under the tire. Pull up on the crowbar, and check with your other hand, on top of the tire, for any sense of looseness. The top joint can be checked by grabbing the top and bottom of the tire, then alternately push and pull the top and bottom to see if there is any movement there.
If you find anything , your on your way to being a professional.