Ok…
…so, like many others, this problem seems to arise again and again for me and I think I've resolved the problem though I fear it may seem like one of those pat-your-head-and-whistle solutions (no real technical explanation) but here goes.
Working with a file that was causing me a problem (no 2D or 3D export of DWG) I copied my model from the existing drawing and pasted it into a new, default settings, SketchUp model space. Set up my view and went to export. Nothing. And then I tried the 3D save and received a "can't save file path" message; for no other reason than to try something I had never tried before I tried again but saved the file to my desktop. Success! In both 2D and 3D.
Being an overly organized student I am often working on models whose file is saved in a deep nest of folders. It seems that SketchUp has problems "finding" or pathing from a model nested in one file path to folders nested in another file path.
I have gone back to the original file I was working with yesterday that was causing problems and it too will export DWGs perfectly if I save them to a more surficial location: Desktop, general Documents folder, etc.
I have also tried this with five other files that I had flagged over the past months as problem files and they too are working now flawlessly.
I'm not a programer so have no words of wisdom of a technical nature to offer but this is a quick try that seems to have resolved the DWG issue for me.
I'd be curious to hear from folks if this resolves their similar issues.
Cheers--Chris