Don't panic just yet. First thing I would try is a change of feed, and some extra vitamins and minerals. Several vitamin deficiencies can present with varying degrees of lameness and paralysis, and older feed or a mis-mixed batch would be the first suspect.
Marek's (usually) takes weeks from time of exposure to symptoms, which is why you almost never see very young chicks with it. Are you in contact with anyone you've sold birds to in the past few months? If it IS Marek's, all your birds are already exposed, and have been for some time, and anyone you've sold to fairly recently could be experiencing some trouble as well, and finding out if they've had any birds come up lame and die, would definitely narrow down the field of what's going on. You don't have to panic the new owners, just do a friendly check in in case you want some breeding stock back from them at some point or something.
Check ALL your birds for condition, to see if anyone is getting skinny (sharp keel). Separate those guys if you can and keep an eye on them for the next few months. If it's a feed issue, and you change the feed, they should put weight back on. If it's Marek's affecting gut or organs, they won't put it back no matter how much they eat.
Should you decide to euthanize another that's lame, have a peek around inside, it's free, and can tell you lots of things about your bird's condition, whether it has Mareks or not. If you can't bring yourself to unbuild a chicken, a report from Guelph (while not cheap anymore) can help solve the problem and help make the decision on what to do next, give you clues on what's working and what's not.
If it does turn out to be Mareks, it's entirely up to you what you do next. There really isn't anyway to keep a chicken from being exposed, short of a hermetically sealed environment. It's my (uneducated, mind you) opinion that occasional exposure to tiny amounts of infected dander builds immunity in many chickens, and is why adult chickens in their prime don't seem to catch it, unless they've previously been kept in very controlled conditions without any wild exposure at all, or are unfortunate enough to come into contact with an extremely virulent strain. There are several options for you to decide from, and all of them are work: whether it's cull all, sterilize and start over; cull only as illness presents and breed for resistance (you will miss some that seem healthy but will sicken and die later on from the lingering tumours or scarring that results) or vaccinate all birds, which ensures that they all carry Marek's, but you may get a few healthy breeding years out of them before their immune system tanks. If you want to sell birds that are old enough to have left the brooder, the last two options aren't great for your buyers unless done with full disclosure. Day olds should be fine though, as long as they weren't hen-raised. If you have hard-to-find breeding stock, the first option sucks donkey
, especially since there's no guarantee that any amount of cleaning, or any period of waiting time will remove the virus from your environment.
The final option is giving up birds completely, but that's one that I personally, can not recommend.
If it IS Mareks, you'll get no judgment from me personally no matter what you decide. I've never seen Mareks in my flocks, but they're potentially typhoid Marys in their own right, and I've taken some heat in the past for not taking a flamethrower to lock stock and barrel.