Shrink data MDF file on server running on low disk space
Your sp_spaceused provides 2 sets of the data:
- DB Size + Unallocated space - These numbers include BOTH: Data and log file;
- Total statistics of RESERVED space for all objects within the database;

I bet your 36 GB of free space are in the Log file.

For real numbers use following query:

SELECT 
    f.file_id as "File Id",
    f.physical_name as "Physical Name",
    CAST(CAST(ROUND(f.Size/128.,3) as DECIMAL(16,3)) as VARCHAR(16)) AS SizeMB,
    CAST(CAST(ROUND(FILEPROPERTY(f.name, 'SpaceUsed')/128.,3) as DECIMAL(16,3)) as VARCHAR(16)) AS UsedSpaceMB,
    RIGHT('  ' + CAST(CAST((1 - FILEPROPERTY(f.name, 'SpaceUsed') * 1./ f.size) * 100 as DECIMAL(5,2)) as VARCHAR(6)),6) AS FreeSpacePrc
    ,  CASE f.file_id WHEN 2 THEN 'Log File' ELSE IsNull(g.name,'N/A') END as [File Group]
FROM sys.database_files as f with (nolock) 
LEFT JOIN sys.filegroups as g with (nolock) ON f.data_space_id = g.data_space_id


Might happen you still have some opened transactions, which hold you from shrinking individual files. See who hols them, close them. Reboot server if necessary. Shrink individual files while nobody accessing the database. switch it temporarily to Single User if necessary:

ALTER DATABASE &lt;Your database&gt; SET SINGLE_USER