Sorry for spamming messages (I can't find an edit post function), but I just discovered that Vurt had updated the Billboards file the day after its initial release and I had somehow completely missed this and was using the original, unfixed file. This is what was causing the issue. Ugh, sorry about that.
SFO's author responded that it shouldn't be the meshes and I suppose he's right as it's not the meshes themselves causing the problems but I guess the way DynDOLOD is reading them? Anyway, I figured I should also mention that this crashing occurred while using both 1.49 and Beta 2.09 (tested on both using Medium and High default settings and also with all lighting effects options) and SFO 2.5b with the 2.5 Billboards. I haven't tested again but a few months ago while playing with SFO 2.3b and 2.3 Billboards and using DynDOLOD 1.48 I had no problems at all. I do have some different mods installed but only the SFO 2.5 Billboards file adds the two meshes mentioned previously and still, deleting them solves the crashing.
After spending the better part of the day installing/uninstalling various versions of DynDOLOD and rebuilding meshes again and again I think I finally found the problem with my persistent crashing outside Whiterun while using the DynDOLOD esp file (unchecking the esp didn't cause crashing). I don't think this is a DynDOLOD issue per se and I have posted about it on the SFO Nexus page but I figured I should post here as well. Anyway, the latest version of Skyrim Flora Overhaul's Billboards file contains two old meshes "tundrashrub05" and "tundrashrub07" which are causing problems. deleting them seemed to fix the crashing.