Did you seal the center section in the housing using RTV? Drain plug tight? How much gear oil did you put in? It should be 2 quarts. Are all of the axle bolts still tight? U-bolts that attach the driveshaft to rear tight? All bolts on the front of the driveshaft tight? What all did you disconnect when installing the rear? I can't think of any reason why the 9" kit would make it hard for you to get into different gears. Sounds like a separate issue.
2010 IOM Camaro 2SS/RS manual trans
(formerly Whipplecharged, then twin turbo, now just a single On3 Turbo)
Magnacharged 2000 Trans Am WS6 manual trans
2010 Dodge Ram Crew Cab Sport 4x4