The issue was caused by a mass deletion call via the API. More than 30,000 items, intention to delete all INACTIVE items in order to clean up the QB database. Problem has been solved by connecting to the QB database via QODBC read/write driver and performing a mass SQL DELETE query via linked table in MS Access on the Quickbooks side, which appears to have been why the sync engine was hanging.
It's still syncing, but now at least things are moving along. In the future, updates of this nature will just have to be made in smaller batches.