As far as I know it is the original 3:07 gear.
It hapens in any gear. Give it some gas and let up and you can hear a slapping noise. I noticed it one day lugging the engine in 1st gear at very low speed, the car would lurch back and forth like there is a lot of play in drive train?
I can't help ya other than to suggest checking all six u-joints to begin with.
I notice that "slapping" noiser of which you speak, come up occasionally as well in mine, but I've written it off in the past, as being part and parcel of the computer controlled devices.
Cars just ain't what they used to be; I can remember lugging an engine way down on the rpms, yet not have it buck and jump around. Now you can't seem to do that, what with the way the TPS and IAC and all the rest of the car is controlled by that ECM (or whatever the latest acronym is).
The first thing I'd check would be the u-joints, then get into the rest of the drive train (bearings, lash, etc.).