I'd see if you can get your prolactin level checked. If your LO is 2 1/2, it's probably normal, and nursing isn't interfering at all. The only thing that might alter that would be if you're still doing a lot of night nursing. They do say that rare women can't get pregnant while nursing, but my thought is that you'd also be experiencing issues with ovulation, etc. I've never found anything that specifies that, though, so it's just my guess. I think a lot of OB's don't have up-to-date info on ttc and nursing, though.
I'm 43, and got pg while nursing my then- 23 month old 6 or so times a day, and still night-nursing. I was planning to cut back a lot when DD turned 2, but got my bfp about 5 weeks prior. I do have a theory, though, that at our ahem...maturity level....it's harder for our bodies to recover from pregnancy/baby-rearing/night nursing, etc. I found when I started really focusing on getting enough sleep, eating my optimum diet, and also increasing my fat intake, it happened. I also read The Infertility Cure, and started doing the acu-pressure that she recommends in there about 2 months before I conceived.
I guess, for me, weaning was something that was on the horizon as a possibility, while ttc, but, I always felt like there were other things I could try first. I also felt like, if weaning didn't work, then I'd have a weaned-early (for her) toddler, and no baby, anyways, so it was my last-ditch thing, if that makes sense....But, everyone comes to their own decision with that, and it just has to be right for your family.
I'm sorry for your loss, and I hope you get a nice healthy pregnancy started soon!