68 Chevy C10 Rear End Gears?

[jsbo1968]

Can anyone tell me what this rear end code is? NF3896838
My Truck was Orig a 6Cyl, W/3 Speed on the Column. It revs pretty high when driving. Feels like it needs to shift again. I want it to be for daily driving mostly and sometimes (very little) on the highway. Motor now is a SB350 w 3 Speed Auto. No Overdrive. I have searched my Door Tag w/ no luck. And the glove box tag didn't have any gear info.

Thanks for any help. JB