Is there a field that can be used to map an Item Fulfillment line item to the Sales Order line item it was created from? I can't rely on the line ID or item ID and it doesn't look like there are any reliable joins between the two.

use the Applied To Transaction Join

To expand on @battk's answer - in searches you can get the line id of the linked sales order line using

Applied to Transaction fields... Line ID

, and on the actual item fulfilment record object this field is called

orderline

.