My fiancée just pointed out to me that I apparently voice the ‘th’ in “thank” and “thanks” just as in the word “this” — which apparently is not in the dictionary! it’s supposed to be unvoiced, as in “think.”
Is this just me? or are there other dialects or variations of this? I feel like I’m going mad!
I don’t think I’ve ever heard (of) that usage. Looks like we’re headed for the Great Consonant Shift.
I definitely pronounce it without voice — as [θ] and not as [ð]. 
Unvoiced here as well (West Coast Leftpondia). The voiced version doesn’t sound strange to me though, even if it’s not listed in the dictionary.
occasionally my pronunciation of certain words gets criticized but never “thank you.”
You must not be putting enough stress on the word.