Make sure the emergency/parking brake is not engaged or in use.
I know it sounds stupid, but it happens and the rotor will not come off.
You can also start the motor with both rear wheels in the air, start the motor, let it idle, put it in second gear then gently part way apply the parking brake.
Repeat this over and over until the rotor comes off.
This will sometimes walk the brake drum or rotor off.
Do this slowly and gently, the truck rear end should be on jack stands with both the front wheels blocked.
Good luck