I can't comment on the ulcer medication. With ulcery horses it is important to remove stressors. Full turnout with buddies would be best, and reducing the joint pain. Once you get the horse out of pain his ulcers will probably be easier to manage.
I have never had it recommended to me to use Adequan IV, nor have my friends/colleagues/various veterinarians and I have never heard of it being used that way, even in Fla. during WEG qualifications where horses were being injected up to 4 times a week.
The risk of muscle atrophy with Adequan IM is very, very low.
Adequan and Polyglycan are also NOT similar in mechanism. Polyglycan is more similar to Legend or HA, but with added glusocamine. Adequan is the only drug proven to improve the cartiledge in the joint.
I think your horse's hocks are a bit far gone for IM/IV shots, personally. His jhock joints need to fuse, either by surgical intervention or time and pain management. There are some veterinarians who are using an IA shot which will help fuse the joint it is injected in, but I have never done this.
Basically you are now into the point of arthritis where you are doing pain management. There are some expensive therapies like Tildren, IRAP and stem cell (A cell may also be available on a "test" or "experimental" basis near you..) which may help, but at this point you would need to bring the joints back from the grave. Right now you need to be encouraging them to fuse. Adequan may help the milder arthritis in his neck and other joints though. And IV Legend or polyglycan can also serve to help these joints out too. Overall, what I would recommend (just based on personal experience, I am not a vet) is to have the horse on full turnout with buddies and that he is encouraged to walk, usually by having the food and water sources far apart. I would also have him on meds to manage his pain. NSAIDs are great if you can find something that wont upset his stomach. I have found Asprin to work and be less upsetting than bute. You should also try to consistently work him as you have been, not that he is sore, but to get him moving. I would also really try to keep up with the IA steroids if at all possible. Try a round of Adequan (5ccs per dose every 4 days for 6 doses) and see if that helps him at all.
Once the joints are fused, he should be far more comfortable.
They say money doesn't buy happiness -- well happiness doesn't buy horses!