Personally, I would have said composing the necessary words to tell Sally...

Both your examples sound unnatural to me. If blind John has difficulty articulating the words verbally, that means he has trouble pronouncing the words, which I don't think is what you are going for. I would use find in both cases: he has difficulty finding the right words to tell Sally/write to Sally how much he loves her.

@JanusBahsJacquet The verb articulate has more than one meaning, only one of which is synonymous with pronounce. And I am sure that blind John's difficulty was not with his pronunciation, but with articulating - meaning having or showing the ability to speak fluently and coherently. (Oxford Dictionaries)

@WS2 Perhaps it's just me, but I cannot force articulate into that sense when used transitively with the words as the object. If the sentence had been “Blind John has difficulties articulating how much he loves Sally”, then that is the obvious meaning—but it's a very stretchy stretch to me to apply that meaning to “articulating the words”.

@JanusBahsJacquet The OED provides several examples, including, 1952 R. Ellison Invisible Man xiii. 221 We need a good speaker... Someone who can articulate the grievances of the people.. But interestingly it quotes another sense of articulate, meaning "to express or convey, especially through non-verbal means". 2006 Independent (Nexis) 24 Nov. 24 This period of European cultural history, when paintings were articulating the first expressions of sexual liberation. But I still think that compose would be a better word for mute John to write.

@WS2 Those both sound natural enough—note that in both cases, the object is an abstract notion that one can articulate into words; you could actually add “into words” into the first (and with a bit of poetic licence even the second) example and it would still work fine, or replace “articulate” with “put into words”. But “he had difficulties articulating/putting into words the words to say…” doesn't work.

@JanusBahsJacquet. It was not my idea to put this into "chat", the system just took me there. It is not chat, it is a serious discussion of how to use the word articulate. What we have done, I believe, is to discover that it doesn't sound right to use articulate with the words as a direct object, perhaps since words are already implied in the verb articulate. What else would you articulate, if not words? It's all a bit like saying he was drowned (in water) when his boat capsized.