I've noticed the same issue with my DX55. I did a lot of research on the 55 before purchasing it, and the steering lock was a concern but a minor one. Mine acts just as described on the many previous posts... It is fine when moving but rarely locks up when standing still and lifting a load. When I am moving stone or dirt, driving into and backing away from the piles as I lower and raise the bucket, I never notice a problem. If I stop before dumping, and try to turn the wheels after I stop before dumping, it can become locked. Not hard to turn, but locked solid. This isn't like an old tractor that becomes difficult to steer when under a load. These are hydro steering and it feels like the steering wheel is locked. If I move slightly forward or reverse it becomes free again and I can tuen the wheel with one finger. It rarely happens to me, and has never happened with a ballast on the back (yet). I'd say it has something to do with the steering being close to it's limits and it cuts out if pushed over that threshold. The ballast reduces the front axle weight so that helps. Wheels in motion reduce some of the friction that reduces the load on the steering, which also helps.
I know this has been discussed on this site many times. It's not abig issue to me. It's someting that helps me realize the tractor's limitations and probably keeps me safer in the long run.