The inside edge of all 4 tires on my 1999 xk8 is worn down to the cords. Outside edges look new?? Where to I look for the problem? The wear is very uniform. Can I put 245x45x17 tires on as replacements instead of 245x50x17

 

I would check your suspension for failed front upper shock mounts (you will find foam bits around the top of the shock), bushings, shocks all around for leaks/bad lower shock bushings, etc. Replace these, then add new tires and an alignment.