Well, since you say it that way, I should chime in. Sorry for the long post. I just completed the entire unpleasant process of replacing my 3rd member because I broke 2 teeth off of the rear pinion gear. That also damaged several teeth on the ring and did some damage to the carrier as well. I purchased an entire pre-assembled 3rd member from Marlin Crawler with the same gear ratio (4.88) that I had before. It's nice that Marlin uses the stronger pinion spacer and a flange with 3 different sets of holes in it, but that's just frosting on the cake. I was happy to buy it from a shop that should know what they are doing in setting up the gears. Then I could also do the complete uninstall/install of the 3rd member in my garage so I know it's done right.

My rear ring and pinion broke while I was on the road doing several unusually fast accelerations, which apparently heated up the gears to the point of weakening them. I rarely drive with full throttle, but I was enjoying driving since I had improved the Split Second tuning on the A/F ratio at full throttle and I put the URD 2.2" S/C pulley back on to make it roughly equivalent to sea level performance despite that I'm at mile high altitude. I was amazed that I could actually squeal the 33" tires in the 1-2 shift despite an auto tranny. It shifts fairly hard due to the IPT valve body mod, which is what I think was a major contributor to the gear failure due to the added spike in torque throughout the drivetrain at full throttle. I had about 80K miles on the gears and some of the bearings in the 3rd member but 150K miles on the spider gears and at least one of the bearings since that one e-locker bearing doesn't come in the master install kit with the Yukon gears that I had installed back in 2002. In hindsight the gears seemed to be setup with a pretty deep contact, (although I'm no expert) but the bearings were worn enough that there was a fair amount of play in everything and it all felt really loose. So I think bearing wear was another contributor for me, unless it was setup a long time ago with way too much backlash.

So about the play in your driveshaft, a 1/2" does seem like a lot. I'm pretty sure I only had 1/4" or less before mine broke - most of that play likely in the spider gears and other contact areas before it rotates the axle shafts, not between the ring and pinion. Both the transfer case/transmission and the rear diff have to allow that much play in order for you to rotate the connected drive shaft 1/2". After what happened to me, I'd think the rear diff is more likely a concern. But I really don't know how much rotational play you should expect at each end of the driveshaft.

When you drain the rear diff fluid, are there much shavings on the magnet? A little bit is normal. Since you feel a clunk, I'd think you do have a problem that needs to be looked at.

One similar "clunk" that I dealt with years ago was fixed by simply tightening the bolts that hold the stock bolts in each end of the rear lower control arms. Make sure those are tight, although that doesn't sound like it's your problem.

My wife previously had a Ford Ranger with a loud clunk in the drivetrain every time she started out from a stop. It had about 75K miles on it. There was a lot of play in the driveshaft - probably 1/2". I opened up the diff cover and saw the spider gears moving around a LOT. I don't know if that info can even be compared to yours since they are very different designs. Unfortunately, you can't just look inside Toyota 3rd member. You might need someone experienced in what feels right to know if it's worthwhile to dig deeper for a problem.