I think I'd insist on an Xray first or ask for a neurology appt but not schedule an MRI until you've met with a neurologist.
Dogs do get both front and hind limb weakness with SM and it can cause limping but no pain. The reason is that the dog just experiences weakness. Leo has this -- he might jump on my lap but has a very poor ability to balance and can just fall over sideways. :
If the weakness is due to SM it will likely come and go. If it returns (and an xray shows nothing, if done) I would ask for a neurology appt. Not for an MRI immediately but for a consult. A neurologist will test for weakness as well as pain and will know what areas to press for example -- pain isn't always apparent until a neurologist tests the potentially affected areas (eg they will push down hard on flashpoints.They will do other tests as well andthe neurologist will know pretty quickly whether it should be followed by an MRI.
Hopefully it is just a sprain or pulled muscle but it would be odd for there to be no pain with something like that. You could also go see someone with an orthopedic speciality perhaps?