Shuddering and slipping

[Rbance]

Hi Everyone,
Im new to SA mini.

I have a 1975 austin 1275cc.
When I go up hill or if theres weight in the car, theres a violent shuddering and then what seems like slipping of something. The result is no forward motion.

Myself and my go to mechanic are stumped.
Ive recently changed out the spline hubs, CV inner and outer.

Any suggestions?
Any help / advice is appreciated

Thanks