CV boots split, especially if you run low or corner hard all the time. Then you get the usual, grease comes out and CV joint siezes up. I replaced both my shafts in the summer as one joint went and i like new parts!
Cost about £60 each on exchange i think. Easy to change just dont strip the threads in the casing on near side when doing up bolts. I heli-coiled mine as one was already stripped and it gives a bit more bite when doing them up.
Do the usual test, apply full lock at a very low speed (empty car park etc) and if there is a cluncking/knocking sound from either hub then that shaft has gone. Best to get em changed especially with the extra power youve now got!