How can I auto-increment a column without using IDENTITY

See more:

SQL-Server

SQL-Server-2008

I'm creating a table with two columns that I want to auto-increment. One column is a primary key, so I'm using the IDENTITY keyword on it. The other column will be used to track the user-defined "sort order" of items in the table. Any time the user moves an item, its "sort order" will swap values with that of another element. However, when an item is inserted into the table, the inserted item should always be auto-assigned a sort-order value higher than any other value in the table. Here's a simplified version of the table creation script:

This is my create table script,

CREATE TABLE [dbo].[tbl_category] 
  ( 
     [id]           [INT] IDENTITY(1, 1) NOT NULL, 
     [category]     [VARCHAR](50) NULL, 
     [last_updated] [DATETIME] NULL, 
     [priority]     [NVARCHAR](max) NOT NULL 
  ) 
ON [PRIMARY] 

Here, i have also mention my stored procedure script which i have done,
here you can see my last field priority, I want auto increment priority field,
above you can see my create table script in this i have already use identity keyword with id,
so, how can i increment with priority field ?

 INSERT INTO dbo .tbl_categorynmpics 
            (category, 
             last_updated, 
             priority) 
VALUES      ( @category, 
              @imageurl, 
              Getdate(), 
              @priority ) 

Thanks,