1995 Mercury Marquis Steering gear box

I just had my steering worked on; replaced Pitman Arms, Idler Arms, Tie Rod Ends, and Adjusting Sleeve. The mechanic said that he didn't think the Ball Joints and Gear Box needed replacing. I didn't tell him that my father-in-law had replaced the Ball Joints about a year ago, so I felt he was honest with me. My problem: A month after the repairs, I'm getting a loud screeching sound when I turn the wheel. A novice mechanic friend of mine says it's the Gear Box they didn't replace. Is he right, and how costly is this repair? Is this a po-
tentialy dangerous situation, or is this something that I can try to ignore? I've already spent $798.57 that was hard to come by. This is the only car that our family has, so it gets alot of use. Thank you!