When I adopted my Yorkshire Terrier mix from the local animal shelter, he was at first very suspicious and standoffish, now he is very pleasant and at ease. He now has justed turned two and we are still unable to leash him, let alone take him for a walk. Once he sees the leash he takes off, and never gives up running away. One time we did get the leash on him, and he went crazy, doing 'crocodile death rolls'. I was so scared, it took a while to even remove the leash for fear of being bitten. It's obvious he was mistreaten in his previous home.
Any ideas?

Personally, (and this is NOT the only way). I'd get him on the leash by offering him a reward and getting the leash on his collar while he is distracted.

Dogs tend to have more common sense than a lot of people do, and except for running out into a road, or overeating they aren't going to hurt themselves. Let him have his fit then when he gets tired offer him another treat and when he has calmed down, THEN pet him.

If you have a really short leash you could put it on him in the house and let him tire himself out.

He's not going to be happy at first but will get used to it if you offer up positive rewards after he calms down.