Standard gearing for the 68 w/ 4 speed was 4.57. Also available is the 4.10, usually found in trucks with automatics. I looked for about a year before I found one locally. What you need to look for is a 64-72 10 bolt full-floating differential. All that you need is the carrier assembly, which includes the gears, pinion cover and trunnion. This will help keep the shipping cost down if you have to have one shipped. The assembly swaps out, it's a simple change.
If you remove the rear cover, there will be 41 teeth on the ring gear, 10 on the pinion. The ring gear should also have the ratio stamped on it.
Sometimes these show up on the 67-72 board, there was one there about a week ago.
http://67-72chevytrucks.com/vboard/forumdisplay.php3?f=8