Loosen the locknut with a box-end wrench, soaking it first with some Liquid Wrench; tighten it no more than a quarter-turn at a time AND TEST IT OUT FROM LOCK TO LOCK!
See, you're most likely doing your adjustment in the center of the travel, where there's the most wear. If you tighten it too much, it'll bind up at full left or full right turn, where there is relatively little wear and things are likely mucher tighter already.

Best procedure is to do like the shop manual says and jack both front wheels off the ground so you can "feel" what you're doing.
Don't get greedy; this isn't a rack & pinion box, so it's never going to feel really tight.