Hi! We're in the same boat. DS will be 4 when we go to WDW in May, but he is already over the 35 lb limit for our umbrella stroller, and I didn't want to spend the money for a stroller which has a weight limit of 40 lbs., knowing he can reach that weight by summer time. The problem is he doesn't want a new stroller, so when we went to BabysRUS and saw a stroller that has the 45 lb. limit, he didn't want to get in it for us to see if it would work. That stroller was about $50, not a name brand, so I forgot the name already. It had the word Euro in the name. I think BabysRUS is having a $10 off sale on strollers through this weekend (2/29) too (though I'm not sure if it's only for the more expensive strollers or not.)
Also, I did a web search and found this website:
http://store.yahoo.com/albee-baby/cotrsa21umst.html
I don't know if they still have this stroller in stock, or if there's anything wrong with it. It's the 2002 version of a Combi Saavy and is $50 (although with shipping, etc...it might go over $60). People on this board had said good things about the Combi, soo... this is just FYI, I guess. I'm a little wary of buying over the internet when I'm no 100% sure that I want the item; 'cause returns may be a pain. I hope you find something!
(I guess the other option of getting DH to go through weight training so he can carry the boy when necessary is out..

)
P.S. Our umbrella stroller seems to still hold, but I wanted one of those baskets on the bottom of the stroller...