I have a 1999 Beetle TDI with a 5 speed manual trans. I thought I had a bad axle shaft now I think it’s the transmission. Everything is fine when driving easy. However under moderate to hard acceleration there is a grinding noise and it stops pulling. It sounds like the splines on the axle shaft are slipping. I taped a string to each axle and made the car slip while sitting in the garage. Neither axle rolled the string around it so I don’t think they are turning. What should I check next?