98 F150 4x4 prob's

I have a 98 F150 with the switch 4 wheel drive with auto hubs.....

the truck will go into 4 wheel drive but as soon as theres alil bit or resistance ont the front wheels it kicks out...

i put it up on jack stands and when u put it in to 4 wheel drive all four tires will spin and then the fornt ones wills top and theres a clicking noise from the trans case...i checked the vaccuum actuuator on the front axle and it stays pushed in when the front tires kick out so iam wondering

IS IT THE TRANS CASE?
OR IS IT THE ELECTRIC MOTOR ON THE BACK OF THE TRANS???

thanks