super easy and you're certainly due. They're to be changed at 60k.
www.oxygensensors.com
OEM Bosch sensors with the right connectors for $119 each or something close to that. If you have a floor jack, you can do it with normal tools and some grunting and at least one bloody knuckle in 45 minutes.
A $10 02 sensor offset slotted socket made it really easy for me.
Change out all four (required) plus your MAF and you should be CE light free for a long, long time. Plus, you'll get better fuel economy and possibly more power if you disconnect your battery afterwards for about a half hour or less. (this makes the DME recalibrate the system, which includes rough idling for a little while).