Doesn't sound like a Firebird code. 68 Firebirds have about 16 different axle codes, 7 start with X (XB, XC XD, XE, XF, XG, and XN) 9 start with Z (ZB, ZC, ZD, ZE, ZF, ZM, ZN, ZP and ZR). Which axle tube is it on, drivers facing the back, or passenger facing the front?
JG is a code for a 2.73 axle from a 1973 X- body (Olds Omega, Pontiac Ventura, or Buick Apollo - not a Nova axle code). Whatever it is it is using the post 1971 GM coding pattern and was assembled at the GM gear and axle plant in Detroit on the 56th day of the year during the night shift.