On my '06 LJ, it's on the passengers side of the frame. On the side of the frame. If I remember correctly, it's near the front the transfer skid plate.
Depending on the year. Dash tag, under left side seat on the raised hump for the seat base is another tag screwed down, grill tag right side of grill on the lower forward pointing flange, and then the obvious on the right side of the frame generally around the area where the trans skid is bolted on.
The frame stamping ranges from the earlier solid numbers like you would do with number stamps to the later ones done with a dot matrix print.