How to stack 2x4 table legs?
It would be helpful if you could add a picture of your desk to your question. The commenters are correct. A work table with 2x4 legs is probably not fancy, and doesn't need a fancy job to raise it. If it were my project, I would either replace the legs with longer ones, or add a 5-1/2" stub under the leg while scabbing a full length 2x4 leg to the inside of the original leg. Use 2-1/2 inch screws and horizontally screw the new leg into both the old leg and the stub. You can optionally toe screw from the original leg into the stub - just like a pocket hole, but less of a process.