You probably won't like this answer because it doesn't help much. This looks like Marek's. But it also looks like other things, too. There is no way of knowing for sure without either a blood test (can be done with a live bird) or necropsy (can only be done on a dead bird, and is not 100% confirmation, just "best guess"). Since there is no cure for Marek's, but there are cures for other things that look like Marek's, you may try treating for other disease.
Paralysis and poor limb control as demonstrated here can be caused my lesions of the sciatic nerve, symptoms of Marek's.
But it can also be vitamin deficiency:
It may be worth exploring vitamin supplementation. Be aware of the differences in fat soluble vitamins and water soluble vitamins; fat soluble vitamins should always be dosed very carefully, or they can do more damage than harm (especially to liver and kidneys).
Botulism can also cause paralysis in chickens. Botulism is ingested, usually via food that has gone bad, but if your chickens free range it's possible that they got into something they shouldn't have.
Other possibilities include: Physical injury and dehydration.
That definitely looks like my Paula Dean. I have read thte links from UNH and Cornell. The only thing that seems odd is that we only saw symptoms for 12 hours and she was dead that evening. How long have y'all seen symptoms? The other question I have is that all our girls are atleast 18 months old. Can I still vaccinate them now?
Sometimes they seem to go that quickly-- If she had the lymphoid tumors, they can grow in major organs, and with alarming speed. The tumors may have been growing and then "all at once" she seemed symptomatic and passed away shortly thereafter. Viceral Marek's often grows lymphoid tumors on the heart, liver, and other key organs than can fail all at once.
You will probably get different opinions on this, so I will try to stick to facts and then give my opinion.
At this time, there seems to be no harm in vaccinating at any time. Meaning, that it cannot HURT the chickens.
At this time, it seems to be questionable if it will actually do anythig to help them.
The vaccines are designed to be administered either in the egg (some commercial hatcheries do this) or more commonly, immediately after hatching. Chicks ideally are vaccinated within 24 hours of hatch, 36 being the cut-off point recommended by the pharmaceutical companies that make the vaccines.
The vaccine is not a cure, and it does not guarantee immunity. It simply gives the chicks a chance to build up a resistance so that if they are exposed to Marek's at a later time, their immune systems can fight it off. Such exposed but vaccinated chickens can be considered "carriers" of the virus, and may shed small amounts of live virus their entire lives, even if they build a resistance and do not show symptoms.
Therefore, it is generally thought that vaccination AFTER a chicken is exposed to the virus is probably completely ineffective.
It is thought that vaccination of older chicks that have NOT been exposed to the virus may help them or it may not. There is no solid evidence but again it cannot hurt.
There is a study that suggests that re-vaccination of chicks (that had been previously vaccinated) MAY help their resistance. How this plays in to all this is uncertain, but it shows that vaccination after the initial 36-hour-period may actually help (Though!! It is always most important to vaccinate during those first 36 hours if it's possible): http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2643530/
My personal thoughts:
Based on my research, I think that vaccination of older birds that have not been exposed to Marek's has a small chance of helping, but it should not be depended on the same way as vaccinating "properly" (within 36 hours of hatch) should be. It should be considered a last-ditch effort if you are concerned about Marek's exposure. I don't think vaccination will help an already exposed bird. All of this being what it is, it's important to note that the vaccine will not hurt the birds, even if it does not help them, so if you think it is worth it then by all means give it a try.
This is, at the time of writing, one of the best prices online and their shipping for the virus is less than other websites: http://www.twincitypoultrysupplies.com/store/index.php?main_page=product_info&cPath=46&products_id=574
The only harm in giving the vaccine is that it may give false hope if not administered properly, and it may "mask" the symptoms if you later get Marek's in your flock. So in the end it is a very personal decision.