Create a .ackrc file in your home directory. For you, it will contain:

  --type-set
  objj=.j

I found this on the ack-grep man-page in Ubuntu. You can set the ACKRC environment variable to give it a different directory (besides ~) to search for your .ackrc. Each line of the .ackrc is a command-line arg that will be prepended to any specified arguments every time you run ack-grep. If you don't want those arguments for a particular query, pass the --noenv flag to ignore the .ackrc. HTH!