I dive with a group of retired firefighters. 20+ years of hose dragging, kicking doors, and running into places where others are running out, absolutely takes its toll on the body. On any given trip, someone is banged up or nursing an old injury. Necks, backs, and shoulders are common. Like you, these guys are true heroes and I'm happy to help them out however need be to get them in the water. They have earned that right.
The primary thing is to discuss your injury with your dive up and see what type of equipment they use and how they can accommodate your needs. Their boat may not be right for you. Odds are though, their DM's and deck hands have seen bad backs/necks before and will be willing to help. Gearing up on the swim platform (if equipped) or in the water eliminates the walk on the boat to jump in. Shedding your gear in the water and having the crew or dive buddy lift it up so all you have to do is climb the ladder gear free and make it to your seat.
Many ops will have wagons or carts to get your gear from the car to the boat. If not, most sporting goods stores have wagons that collapse down for storage.
As far as gearing up in the water, if it's calm, it's prettty easy. An afternoon in a pool will get you squared away. Quick release buckles on at least one of the shoulder straps will be your friends. A Velcro cumberbun on your bcd is also a good idea. If you dive a BP/w. A comfort harness will have the buckles and maybe even a quick release buckle on the waist strap as well. Set your gear up to meet your needs. I get my bcd on barely and start my descent and finish snugging it up. That works for me, but find what works for you.
On normal 6 pack type boat, passing your single tank gear up is no big deal. Some even have a door that lifts out to make it even easier.
In rougher seas, your dive buddy can make it much easier to gear up in the water. Even then, Some days may not be for you. I think the hardest part of all this will be finding the op that will work with you and has the boat that fits your needs the best. A 5 foot drop to the water and rickety ladder with no hand rails may not be the op for you.
There is training for any need, ask some local instructors or divemasters about helping out, Any with experience working with Diveheart may be particularly qualified. There is always the option of hiring a private divemaster as well and let him or her know what help you need.
No need to give up diving, you just have to find an op with the equipment and crew that works with you. Thank you for your service, thank you for your sacrifice, and if you ever find yourself in central Oklahoma the first beer is on me!
Good luck,
Jay