I suppose it would depend on the type or level of intellectual disability. Often I think intellectually disabled people understand a lot more than we give them credit for. I have no idea what your uncles are like, but I'd probably just try to explain it to them like you would any other family member, and just expect that it may take a bit longer for them to switch over to the correct pronouns and your new name.
I also have an intellectually disabled uncle (I have no idea what he has, he was born back in the days where it was taboo to admit a child was born that way, so the story from all the old folks is that he was never the same after being hit by a car, or others say he was never the same after being electrically shocked), I am not out to my family but I would expect that he'd eventually get it. He just often likes to repeat stuff multiple times to others, so I could imagine him saying to other family members "S's name is Henry now, yes, she says she's Henry now." That would go on for maybe a few months until something else gets stuck in his mind. I think if I started medically transitioning, the "he" and "him" may eventually just fall into place.