I've used help em up harness for Tanner when he was alive and now use it again for Yukon. They have it set up where you can use just the front or both the front and the back if the rear end needs assistance. It's well made and you can use it in certain circumstances or more full time if needed. Back end weakness is very common and there are PT things that can help strengthen the weak areas. It would be good to touch base with a PT person and of course a Vet visit to pinpoint the issue (spine, arthritis, soft tissue, etc.).
Yukon is on Denamarin too and needs to have NSAID blood work drawn more frequently to monitor liver enzymes if he's on an anti-inflammatory meds. There are other meds used in combination with an NSAID or steroid, which include Gabapentin, Tramadol, Amantadine, Robaxin, etc. The cocktail approach is usually more effective than a single drug, but the NSAID/steroid are the ones that need to be monitored closely for clinical side affects and follow up bloodwork.
There are also other things that can help like adequin injections (typically for arthritis or joint probs), laser therapy, acupuncture and a host of other PT modalities (massage, ROM, aquatherapy, strength training, balance work, etc.).
Bob