Not entirely true, this is a ZH code 4 pinion nodular out of a 68 with 3:55 gears, saf-t-trac. My cars build sheet says it had a ZH code 3:55 originally. Im' not sure what a YH is, but as stated, a Firebird had leaf springs instead of coils.
68GTO4004Spd: you are correct; In 1968, the ZH axle was a 68 4-pinion, 3.55 Safe-T-Trac. But there is no listing in the 68 axle charts (from the Factory Service Manual) for a YH code in 1968. The Tempest/LeMans/GTO 3.55 open carrier in 1968 was a WH code.
shadetreegto: In 1969, the only YH axle listed for any Pontiac application is the Firebird 3.55 Safe-T-Trac. There are later axles (1971 and 72) for full-size Pontics that also have the same YH code, but those axles would have date codes ending with a "0", "1" or "2".
A YH code axle with Safe-T-Trac and a G148 date code SHOULD have leaf spring mounting brackets, not coil spring perches. It should have an overall width of 60.00". If your axle has coil spring perches, it is probably a unit that has been modified in the past.
I'd check the overall width and compare it to the dimensions in this chart:
Rear End Widths
If it's still not evident, you might want to post some pics.