Well, for me, I've been monitoring my blood sugar intensively for a while. I started when I worked for a nutrition and glucose monitoring company (as a test!), found out I had really anomalous blood sugar dips, and confirmed it with finger stick and another blood sugar monitor. So for me, the answer is, yes, the "sugar crash" postprandial is actually a dip in blood sugar - not to a hazardous level (e.g. passing out or seizure) but to a very uncomfortable level, with epinephrine and the shakes. (mine has gone down to 45mg/dl at worst)
For many people, it may not be, I can only speak for myself. There's another thing, called 'idiopathic postprandial syndrome' which is essentially the symptoms above, but without actual low blood sugar (<60mg/dl), which some people think is another form of insulin resistance, where your blood sugar is normal but your body "wants" more sugar in the blood.
Talking with endocrinologist, they say that the insulin sensitivity for most people is much higher in the AM and daytime than at night, so it makes sense that you might have more problems in the afternoon, but you should probably talk to a doctor rather than taking my word for it!
It's often difficult (in the US at least) to get primary care and endocrinologists to take you seriously if you are not actually dying of diabetes or passing out from low blood sugar - this is where dipping into the realm of concierge medicine can be helpful, or at least, it has been for me. They are often much more willing to investigate thoroughly.
