I have added a 100A 240V sub-panel to my detached garage. I ran 1.5" conduit, 42 ft worth and buried it 18". Here is my mistake...everything works but I think this is a code problem: I pulled 3 #2 THHNs, 2 hots (120V) and 1 for the neutral. At the panel, I have separated the neutral and ground buses, bonded the panel box and drove a ground rod. Electrical geeks in my office said I should have pulled a ground along with the 3-#2s. Ok, like I said probably a code issue but everything works fine.
So the questions to the group are:
1. What's the REAL risk?
2. If the risk is real or I might get hosed on inspection when I go to sell the house, what is the best fix? A direct buried ground? Seems pointless if I have a ground rod.

Thanks in advance.

ground clarification

Two different types of grounds

Thanks for an explanation

Grounds

Pulled...