How did the animosity between Loki and Heimdal start? I'll admit my Norse mythology is rather rusty, but I do recall after an incident these two particular gods saw each other as mortal enemies and would eventually kill each other at Ragnarok. What was it that turned them against one another (I vaguely remember a story where Heimdal caught Loki stealing a piece of jewelry from Freya and kicked his ass over it but the details have eluded me)?

You are thinking about a story referenced in the _Húsdrápa_ , which Snorri excerpted in his _Edda_. It plays out more or less as you described, except Heimdall does not catch Loki in the act, but is asked by Freya to retrieve Brisingamen. The only detail to add was that evidently Heimdall and Loki fought as seals. That is really all we know about it.

Further than this, as have already been noted, Loki and Heimdall will fight and kill eachother at Ragnarök. They also interact in the _Lokasenna_ , a poem where Loki shows up at a party and insults the other gods. Heimdall does not seem to be under more fire than his fellow gods.