All the ones I've ever run across that 'shake-a-leg' are due to the sockets themselves beginning to come loose........it would be a rare event for the legs to actually be wobbling around in the sockets, as far as I've been able to tell........
I have had some legs that were loose enough to fall out when the kettle was moved or picked up & fixed that by setting a small strip of soft lead alongside the leg as it was being set back in.....aluminium foil would work also, if that's actually the problem, but if the whole kettle wobbles when gripped by the bowl handle & jiggled back & forth, I'm pretty sure it's the welds starting to loosen on the sockets......
If it's getting bad enough, either get out the drill bit, or talk to Duke about JB Weld......