I feel like a moron, but after reading everything I could find (and discovering that the RPO decoder wasn't working), I still can't figure out what rear end I have in my 84 T/A ... originally 305HO (L69), RPO codes are: GT4, G80, Y06.

I'm sorry if this is a stupid request, but I'm not too conversant on this and my son is going to drive me nuts ... I would check to see if its a 9 bolt or whatever, but I can't get under the car (too much snow). Any and all info you can provide on this rear end would be GREATLY appreciated!!

 

I would think it's the 3.42/3.73 9 bolt

 

9 bolts were not available until 87 so unless someone has swapped the diff, you have a 7.5" 10 bolt

The 1984 L69 had a couple of options for gears.

Manual transmission has 3.73
Automatic has 3.42
Automatic with the G92 RPO have 3.73

GT4 is normally associated with 3.73 gears
G80 Means you have a posi
Y06 is a mystery. GM always says it means a 9.5" ring gear but that's an RPO for 3/4 ton trucks. No car ever came with a 14 bolt truck diff.

Chances are you have a 7.5" 10 bolt with 3.73 gears and a posi.