95 Silverado 5.7 automatic C2500 Replacement of speed sensor correct 8mph error in my speed. At 60 mph reads out at 68. Checked with GPS.

SOURCE: Road speed sensor

So the diagnostic code says the speed sensor is bad in the transmission? Make sure all the wires are tight to the transmission and in good shape. If they are you probably need to replace the sensor. If the transmission controller doesn't get the correct speed values it would cause some problems shifting. Wiring to a transmission can cause faults since the wires are subjected to heat and vibration. If you can't find any problems with the wiring or the wiring connectors it just might be the sensor is bad (stuff fails).
I do have to say that I am not sure what G-68 means. The diagnostic codes are usually five digit numbers with maybe a letter.

SOURCE: High speed vibration

could be the tire has wobble so rotate the tires 

SOURCE: location of speed sensor on 2004 silverado automatic transmission

the speed sensor is located generally on the tailshaft housing or in other words where the driveshaft slides into transmission has a small black sensor with a couple of wires going to it. those trucks have a bad problem with the cluster going out on them though. more than likely that is going to be your problem. you have to take cluster to dealership (new one) and they have to burn the current mileage into it.

SOURCE: speedometer working some now on

check the Transmission Fluid I do Believe that the Sensor For the Speedo is in the Transmission

SOURCE: my speedometer on my 2005 Chevy Silverado is not

did you put aftermarket tires on it? or
check the speedo sensor on the tranny