Gt1 axle rear, 2.56 ratio
GT2 axle rear, 2.29 ratio
GT4 axle rear, 3.73 ratio (dup with 5 x 1)
GT5 axle rear, 4.10 ratio (dup with gt8)
GT7 axle rear, 3.33 ratio
GT8 axle rear, 4.10 ratio (dup with gt5)
G80 is the "Eaton Locker" aka "G80 Locker". Many consider it the best differential made (arguablly). If I were you, I'd consider fixing it rather than replacing it. If you must replace it, try to get a good 3.73 G80. They are great.
Check out http://www.youtube.com/watch?v=AjymmXvBYsI