B.M.
Hi S.,
We had a very similar issue. My son (I also have 2 boys) would only fall asleep either at the boob or if we put him in his swing and for the first 6 months of his life he ONLY napped in his swing.
Anyhoo . . .what I found was that I would nurse him and as I carried him to his room, I woke him up enough for him to realize he was being put into his crib and he got used to falling asleep on his own.
The other thing that we did was we purchased the fisher price ocean wonders aquarium thing (gosh, I think that's what it is called - minus the "thing" in the title, of course):-) He LOVES it. It really helped soothe him and kept his attention while I walked out of his room. He is 15 months old and still nursing prior to naps and goes to sleep on his own - with his aquarium music.
Good Luck.
B.