Well, I have been hunting Grurn every chance I can get. I have formed a couple small groups to turn the cave to the Grimling's favor, and have camped him solo several times.
I've noticed that he drops the earring randomly about 1 in 4 times. I've noticed that Grurn's pop runs very hot and cold. The fist time I got one earring in a short time. The next time my group got four earrings in about 3 hours. Since then I haven't seen an earring.
I was thinking that Grurn had a spawn-time window where he popped more frequently. It seemed to me the first two times I camped the caves, he popped about every 2-3 kills of his placeholder (the Tribal Leader).
Recently, however ....well ok NOW actually (I am on other PC camping him as I write this - talk about boring!). Anyway, I originally thought he had a window and I believed it was between 11pm and 1am EST. On two separate occasions he popped about 1 in 3 during this window. At other timnes he popped about 1 in 15 times. It was a dramatic difference and it FELT like more than just stupid randomness.
This may be true, but I can confirm that this window I speculate about is not on a 24 hour cycle. I camped him yesterday for two hours and now I am on my fourth hour tonight.
His placeholder is replaced every 11-12 minutes. Grurn seems to pop randomly. Overall I would estimate his spawn chance at about 1 in 10 and the earring drop at about 1 in 4.
His rate and the earring rate may have been ratcheteed down, I am not sure. It sure feels like he's spawning less and dropping the earring less.
Ghowlik (the named sonic wolf), who spawns in his exact spot is reportedly dropping his rare loot much more rarely so maybe the frequency of these have all been nerfed.
Fazool Thunderwood
51st (atm) Druid on Xev.