The first thing I check no matter what the horse, if they're bucking and it's new I check the tack. Particularly the saddle. Horses bodies change season to season and year to year, the saddle that fit her a month ago may not fit her now (or need special padding). So I would immediately check her tack.
The next thing that occurs is that she has arthritis in her hind end, how severe? Lunging at the canter IMO is never a good idea, it's very stressful for their joints, especially one with arthritis. I'm sure she only throws her fits when asked for a forward trot or to canter.
When you lunge her is she in tack?
If she bucks without tack I'm going to say it's the arthritis, if she only bucks with tack the answers obvious.
If it is the arthritis that's the problem don't give up hope, there's a lot you can do. There are more than a hundred joint supplements out there, MSM and Glucosamine being my proven favorites. There are also injections you can get, but I'd be wary of those and decide whether or not she really needs them, because once you start she'll need to be on a regular cycle for the rest of her useful life.
What are your goals with her? Can you take riding real easy with her for a while? Movement is good for arthritis, but not work. So light riding, if the arthritis is in her hind end, let her ride out on her forhand. If you're an english/dressage rider I know you'd typically want to see her moving from her hind end, but that's probably the problem.
I would not be punishing her for these bucks until pain is ruled out personally, particularly not with lunging as that could only be making her pain worse. I personally would not lunge this horse anymore, once they're arthritic it's really no good for them, just hurts. If you feel the need to exercise her a little before getting on long line her around the ring - this will get her thinking too, not just worked up ;)
good luck, keep us posted
ETA: Did the vet look at the horse and say they're just being stubborn? Despite the Arthritis? Did they check the tack? Personally I'd be looking for a new vet if they didn't check everything before calling them just 'stubborn'.