Jen, I feel your pain. Mandy started licking her front legs about a year ago and slowed down noticeably. She concentrated on her shoulders, but would also do her wrists. I had Dr. Kellogg do a couple of sets of x-rays on both legs as we were both concerned about...well, you know what. Nothing showed on the x-rays. We tried NSAIDs, which helped a bit with her limping, but didn't slow down the licking. I also talked with Dr. Kocen at Southpaws, who adjusted her acupuncture to help relieve the obvious tension in her front end. That helped some, but not completely. Because Mandy was also showing some hind end weakness, Dr. Kellogg suggested that I try working with Carol Wasmucky, the PT. Carol was able to isolate which muscles were causing her the pain and we now do exercises to help along with cold packs, a TENS unit (that I already had), and massage. I have noticed a difference. She's 11.5 years old, so she's still a bit slow at times, but she seems more comfortable. The licking has also been reduced dramatically, which is a good thing; that slurping sound makes me want to scream. When I reported the positive changes to Dr. Kellogg, her response was "Carol's hands can work magic."
If you wanted to look at a possible environmental factor, like a grass allergy, the first thing I would do is to start carefully wiping his legs down with sensitive skin baby wipes after every trip outside. That would help reduce any allergens that remain on the skin. We had to do that with our family's dog when I was in high school. It didn't stop the issue, but it helped.
Good luck in getting Zuri more comfortable.