Scott... I can relate, as a couple of years ago, I had the EXACT same issue... left hand thumb, 'locking' into place and couldn't move itself out of lock without the other hand straightening it, there was some considerable pain playing. I guess mine got more severe than yours has so far. Hopefully, we can catch it before it gets worse.

I went to see a physician about it, and basically got three options. Firstly, immobilize it as much as possible with a full hand thumb stabilizer, and try anti-inflammatories off the shelf. The next step was cortisone injections in the hand. Mind you, when asked about effectiveness, my doctor didn't say he guaranteed they would work, and spoke of the next step, which would be surgery on the hand to shave away at the tendon that was getting stuck in the tunnel. It didn't sound very promising.

Being a cheap (and cautious!) SOB, I opted for Plan A... But saw no improvement over a couple of weeks, possibly it got worse.

Then I talked to a guitarist friend of mine, and he said he also had had joint and tendon problems, and had used a homeopathic remedy called Turmeric Pills (yes, the yellow spice used in much Indian and Asian cuisine). You can get these down at most health food stores, and he said the best ones were paired with a activator called Bromalain and Boswellia. He swore blind by them, so I went and got me some (probably about $25 for a week or so's supply) and started taking them. I didn't see any improvement for a few days (the inflammation had got pretty bad, being aggravated by playing 5 nights a week and some days!) but he told me to hang in there...

And what do you know, after about a month or so, the clicking stopped, the joint worked normally again, and I haven't had a recurrence (even though I stopped about a month after the problem went away) since.

I encourage you to do some internet searching for the effectiveness of turmeric, and hopefully, you can avoid the pain and expense of the doctor, and try healing it without injections and steroids, or god forbid, surgery.

It worked for me, and mine had got much worse than you are describing...