Insert Does Not Increment Primary Key Correctly
The application is our ERP and was purchased from a vendor. Their support does not extend to SQL help so I have no way of seeing how it is doing the insert. There are no sequences in the database at all either.


You could run a trace using SQL Profiler to catch the exact statement that your application is executing. 

Also, a slight alteration to Scott's suggestion, I wouldn't use a TABLOCK, instead, something like:

BEGIN TRANSACTION
SELECT @customer_salesrep_uid = max(customer_salesrep_uid) 
    FROM YourTable WITH (XLOCK, READPAST)...


This should reduce the risk of locks on the table.