crt.5 rear drive shaft problem

[RUSTED]

i just got a new rear drivshaft but im still having issues, when i use the spacers move the drivshaft a bit more in the cup , the driveshaft always falls of i the end , and if i didnt use the spacer the driveshaft reaches the cup but just spins inside , im beginning to think that the cup the worn out , gonna replace it today , and wat is a gd diff oil for the center diff if im using rcm chassis and 2.2 tires??