My S80 vibrates under acceleration and seems more noticeable at high speeds, on cruise up hills it shakes, on the level its fine, I think its an axle, how do I know which axle to replace

 

Take the car to an alignment shop and they will tell you what parts are bad. You can always jack the car and try to shake the wheel, up down, left right, and see if it whiggles. I'm not sure what parts are bad if it does but at least you'll know which side.

 

Before you start replacing axles or rather drive shafts, how many miles do you have on the current struts? Vibration of that type is usually a result of bad struts, not bent drive shafts. A bent drive shaft is a rare animal. Struts fail or rather wear out and any inbalance in the tire (which is normal) will cause vibrations at differents speeds and situations. The ball joints on the S80 probably need replacing as well if they are original but this all depends on mileage.

 

Check your tires first. a bad tire can cause this. you can rotate the front tires to the back to see if it fixes it. but if you jack up the car shaking the tire up and down will tell you if you have a bad ball joint if it has play then a new ball joint is need. left and right play will tell you a bad tie rod (must look at inner and outer). look at the control arm for any play during both for play in the bushing. if you grab the cv joint at spindle and pull it in and out and hear clunking noise it will be a bad cv joint thus a new half shaft. usually a torn boot will get sand in it and destroy the joint so first look for a tear in the boot. if you hear a clunking noise when turning it may mean a bad bearing on the fwd strut which is at the top.

 

try shaking the strut too once you have it jacked up to check upper mount. have a jack stand in place before you start shaking things though for safety reasons. you can't go wrong by grabbing anything you see and shaking it to check for play. anything with rubber to metal bonding needs to be checked while you are checking