My 11 yr old jack all of sudden does not want to go out for walks. He still plays actively with his ball in a park setting, but when I take him to go for a walk, he won't go. He has very strong trembling in his hind legs. Is this serious, should I be worried? He still eats etc

When was his last vet check?
Perhaps he has started getting arthritis or patella problems which is why he doesn't want to go for walks.

Did anything happen to him on one of his walks?
A dog barking at him from behind a fence that may have scared him or something?

He went to vet last Sept for checkup and was put on glucosomine. Doesn't seem to be much of a help with the trembling, which is getting me concerned as it seems to be more pronounced now. That's the concern.
Nothing has scared him, he used to love going for walks. Will basically go to play with his ball for a bit and then he's done for the day.
WHat is patella? Thx very much

WHat is patella?
patella = Latin for knee

Ahh yes sorry, the patella is basically the knee joint on a dogs back leg, in smaller breeds they can have a lot of trouble with them slipping and causing pain/discomfort when they move.

I didn't know it was latin though, thanks WonderGirl :D

Seeing as how the vet put him on glucosamine I would be leaning towards and arthritis problem.
It wouldn't hurt to get him checked out by a vet again, my mother in law gives her dog mussell extract for her yorkies arthritis and it was almost like a miracle cure. The dog went from moping around and could hardly walk to bouncing and playing again in a few weeks.