SQL Query with optional WHERE clause parameter for date range?
Assuming that your start date and end date are compared against DlvryDt in DeliveryOrder table, I suggest to use a very low date value for start date and a very high date value for end date in cases where these date variables are null. Please see following query. 

SELECT  do.OrdrNmbr AS 'Order Number',
        mb.BOLNmbr AS 'BOL Number',
        do.DlvryDt  AS 'Deliver Date',
        mh.Text3 AS 'Truck',
        do.DlvrdQntty AS 'Gallons Delivered',
        ba.BANme AS 'Carier Business Associate'
FROM dbo.DeliveryOrder AS do WITH (NOLOCK)
INNER JOIN dbo.ManifestBOL AS mb WITH (NOLOCK)
    ON do.OrdrNmbr = mb.OrdrNmbr
INNER JOIN dbo.BusinessAssociate AS ba WITH (NOLOCK)
    ON do.CrrrBAID = ba.BAID
INNER JOIN SRA.dbo.MovementDocument as md WITH (NOLOCK)
    ON md.MvtDcmntExtrnlDcmntNbr = 'DOD' + CONVERT(VARCHAR, mb.OrdrNmbr)
INNER JOIN SRA.dbo.MovementHeader AS mh WITH (NOLOCK)
   ON mh.MvtHdrMvtDcmntID = md.MvtDcmntID
WHERE mb.OrdrNmbr = @OrderNumber
and do.DlvryDt &gt;= ISNULL(@StartDate, '19000101')
and do.DlvryDt &lt; ISNULL(dateadd(day, 1, @EndDate), '21000101');