At least here in the USA, UPS trucks get used until they are dead and then they pull the parts to use for repair of other UPS trucks and they dispose of the body (crushed). UPS uses their own mechanics - not outsourcing repairs like FEDEX does.
I've heard that the only people they release used UPS trucks are sold to local police or federal agencies for use in surveillance purposes... no one wonders about a UPS truck driving past multiple times and the fiberglass body (on the US models) is perfect for allowing internal antennas, etc.