I personally have recurring sacroilleac pain (HLA negative for AnkSp), but it comes and goes every month or so--not so persistent. I haven't heard of degenerative arthritis--but I haven't looked--being related to crohn's. Usually more rheumatic and migratory.
My cousin is a tall guy (6'7'', 32 yrs) and he was recently told he has the back of a 60 yr old. The only guess they had was attributed to a childhood sleighriding accident!?
I would personally supplement (at least) with Vit D, K2, and Calcium/minerals with regards to possible crohns malabsorption. Maybe drink purified water by getting a good RO filter, too. If you can handle it, I would definitely build up all those little muscles that support the spine by doing a variety of yoga exercises. In some cases moving around helps and if you don't use it, you lose it, so to speak.
I commented on the Army connection because troops are coming back with depleted uranium poisoning and many exposed troops, including a high school peer of mine, start out with fusing bones or bone cancers (often in the spine)--years after they come home. DU munitions, when fired, release a Ur-oxide vapor that, when inhaled, lodge in the lungs for awhile before the alpha particles spread to reproductive organs and bones to wreak havoc. Fortunately, you probably have nothing to worry about if it has been 5+ years and you weren't firing much DU rounds.
Repetitive lifting or some sort of traumatic injury, I believe (but don't know), is what causes most degenerative arthritis.