It appears to have been a minor hiccup - I've tested a 50MB file and it's now working (Flash uploader). I'm just testing a 500MB file to see how large items are handled, and if that fails I'll try your suggestion and report back.
Are you on a shared hosting account or VPS/dedi?
If shared, have your hosting provider check the error logs (server and PHP).
If on VPS/dedi... then did you set it up yourself or have someone do it? And are you using a panel to configure it?
I had someone else do it. But do have access to the server. It is a VPS.
They just did it all as a part of one folder, instead of setting up its own cpanel, which made it confusing to me as my host kept saying go to Xenforo, but I knew Brogan was correct.
I did get it resolved and was on the server end. I appreciate everyone's help!