There could be a few things.
First, check all your tie rod ends, there are four, a little here and a little there can add up fast. Lowes sells them for about $11.00 for two.
Second, your steering box is very well made. There is a cam follower that will wear from contact at the same point all the time.
Take the locknut on the follower, loosen it and turn the follower 1/4 turn clockwise. This will put a new surface against the worm gear.
One of these two (or both) should fix you up. Don't forget to grease it.