I think the OP has a situation much like mine. I have a coop that is just fine for the chickens, and a run that is just fine, but I would LIKE to be able to put them out range on our huge lawn during the day. I can't free range them--no fencing, lots of hawks, eagles, and free-running dogs. So what I need is not necessarily a tractor, but a secure portable day run.
My plan someday is to build a portable mini-house, that could go on a little hand pull cart. I would load them up in the house in the morning, and pull it down to "dock" it with the day run. At night I would herd them back into the house and up to the coop. I know from experience with the chicks that it is super-easy to train them to load up into a portable crate. It just has to be big enough to handle the full grown hens (I can't carry it!).
The advantage of this method is that it would cut a lot of weight and construction off of the day run, so it could be larger and more portable. Plus I feel they are more secure in their coop. If anyone has done this I would love to hear about it!