No don't think its gears the only way the speedo will change is with new differential gears, tires, speedometer gears. if you didn't change the gear that plugs into the transfer case, the rear end or tires, the output shaft on the transfer case has a different # of speedometer teeth on it hense the difference. What I'd do is change the speedo gear they are only a couple of dollars easier to change than the transfer case output shaft. You can either use a little math to figure out what new gear you need, or do trial and error. If you can count the teeth on the new transfer case/ old transfer case, and bring the speedo gear to the chevy dealer and they should be able to tell you. Or maybe there is a # on the new transfer case or if you know what it came from and bring that to the chevy dealer and tell them the rear gear and tire size.. I'd personally check the number of teeth then go to I to 4low.com
Blood, sweat, tears, and lots of money is what keeps my K5 alive.