Dave and Michael are correct--there is no need to replace the half shafts prophylactically. Only replace them if the boots are shredded or if joints are binding or making threatening noises. Even if the boots are torn, the boots can be replaced economically by a rebuilder (they have the very expensive crimping tool for the boots) if the joints are in good shape and you'll be good to go for many more miles.

What fails with the rebuilds is the joints themselves. The races are machined oversize to accomodate oversize balls, but in the machining process the joint loses its hardened surface treatment and thus they are quick to fail. There is a reason why they are only warranted for a year.

__________________
Richard Detoy
'84 300SD
'76 Moto Guzzi 850 Le Mans