I feel for you, I'm in a similar situation. I'm too deep in to fade or disappear. However I do expose as much as possible, put my question marks behind certain viewpoints but in a quizzing way which seems to be great when you're in a group, you're considered being spiritual and studying 'the deep things' and in case somebody decides to investigate further they'll most likely start to understand and find themselves things that are going against all logical thinking.
I'm one of those persons people think is doing great, being blessed, getting Holy Spirit etc. etc. I'm up for promotion actually (the gossip mill is abuzz about it and all the numbers and little points they 'counseled' me on are starting to look good) and it will be really funny for me personally to see an apostate promoted into a position that is supposed to be decided by Holy Spirit itself.
If you're looking to DA yourself, don't do it in a letter that nobody will ever hear about. Go out with a bang - do it in a talk. Work through your subject logically and expose the flaws where the WT goes against the word of the Bible (such as it's viewpoint on Blood or the position of the FDS) by the end of the talk you will have a bunch of confused looking people especially if they respect you, they might even follow your reasoning straight out. The damage control after that is going to be even funnier most elders are not all that educated in the truly 'deep things' of the Bible to even understand what you brought and you might even get away with it if you are subtle (don't call them out) and don't announce yourself to be disassociated.