I tried looking up more info on it, but have come up short. There are some size differences, but that is to be expected with a species that has a wide distribution. Reindeer were imported here for ranching purposes, I am as yet unclear as to why reindeer are prefered for ranching (My family name is the name of the land where my great grandmother and grandfather farmed reindeer). The State University in Fairbanks was also doing some hybridization between caribou and reindeer, jokingly calling them "ReinBou" or "Carideer". As with most state projects, the point of their experiment escapes me.
I will try to come up with more info on the subject if I have the chance. Any way you shake it, they are extremely similar (and tasty..).
YeK