K.L.
Sounds like a muscle tone issue, not an inner ear problem (unless she has an existing condition that involves her ears, it's quite unlikely). A head MRI would have shown any structural/fluid related ear problems if there were any present. Often, regular medical doctors and pediatricians do not recognize "slight" muscle tone issues with children, and since your daughter is only 18 months old, she is still within "normal" range for reaching the walking milestone - in general, your pediatrician would probably not address her late walking issue unless she reaches the age of 2 and is still behind in her gross motor skills. If she had other obvious developmental delays, a muscle tone issue would have been recognized by now, but from your description she is typical in every other way. However, you are the mommy and will notice that something is off FAR before your pediatrician would, so ask your pediatrician for a referral to a neurologist. Here are some very brief symptoms of low muscle tone (hypotonia) and high muscle tone (hypertonia); look them over and see if any of them seem to fit your child.
Hypotonia, or low muscle tone, is associated with postural limpness, floppiness, or a heavy feeling felt when another person passively moves the limb. Usually, children with low muscle tone lack ligament, muscle, and tissue resistance, thus providing inadequate support to their joints, leaving the child hypermobile, and often inappropriately described as being "double- jointed".
Often, hypotonic children appear inactive and passive. This is because they lack the proximal control (strong neck and trunk muscles), needed in order to move within or transfer to and from positions. They tend to maintain postures that provide them with the most support, however the least function. For example, when a child is on his/her belly, often they will assume a typical leg posture referred to as the "frog posture". Positioning their legs further away from their body provides them with a wider base of support, versus if their legs were positioned closer to the body.
Hypertonia, or high muscle tone, refers to an increase in resistance of a muscle to passive stretching, (when another person moves the child's limb). Often times, attempted voluntary movements of distant or opposing muscle groups trigger hypertonia. For example, if your child attempts to reach for a glass, increased tone may kick in, and he/she will not be able to straighten their arm in order to reach the glass, because the muscles that bend the arm have not relaxed.
As mentioned earlier, the brain controls muscle tone, and when the signals from the brain to the muscles are not working properly, tone is altered. This increase in tone can cause many issues for the child, including deficits in motor control, motor planning, balance and coordination, leading to difficulty in functional mobility, dressing, and ambulation, to name just a few.