Hydraulic cylinders would probably be a good place to start, just based on anecdata from seeing equipment down for long, expensive periods of time as cylinders go off for rebuild/reseal/resleeving.
[1]In fact I was involved on a project where some thieves took a sawzall to the equipment storage conex over the weekend and stole every bit of GPS equipment they could get their hands on. They were pretty quickly tracked down when they tried to offload it though.