1978 ford f150 having troubles with tcase and tranny?

Put rebuilt tranny and tcase in recently, when I put the truck in reverse or drive the rear driveshaft turns into drive and reverse but doesnt want to spin the rear axle. With the front hub caps locked the truck will drive with just the front driveshaft powering the truck but doesnt go into reverse with the hubs locked. Any advice is greatly appreciated. Its a Np 203 transfer case and a c6 tranny