They should always be replaced in axle pairs, but can be tested for runout to see if they're bad or not. Or you can jack up the front wheels, grab ahold of them at the 12 and 6 oclock positions and try to rock them inward and outward and see if there's any play. Although this method doesn't always indicate a bearing that's bad..
Definitely diy-able. But I'd suggest having a torque wrench handy, although it can be done without if you're one of the people that uses the German method (goodandtight/gutenteit)..
Do you know what brand bearings they installed? I'd go with Timken replacements if you have the dough, but I installed 1aauto ones after getting some not-so-great replacements through Detroit axle.
I wouldn't expect leaving your center caps off would kill the bearings, usually just rust out the axle nuts, but I could be wrong on that.
Pretty simple, just
If 4wd, remove the axle nuts with the vehicle on the ground and wheels on.
Remove your wheels,
Remove your brake calipers and pads. 2 14mm bolts
Remove your brake caliper mounting bracket 2 21mm bolts
Remove the brake rotors
Then open your hood and disconnect the abs sensors from the vehicle side. You should be able to find their location by tracing the abs wire through the wheel well into the engine compartment. They will be held onto studs with a push down retainer. Simply pull them up off the stud. Then to undo the connectors, go slowly and be careful. They have a red tab on them which must be pulled backward. Then there is a tab on them to push in, and pull them apart.
Once the abs sensors are disconnected from the vehicle side of the wiring harness, go back down to the wheelwell.
You'll have to pop the grommet out towards you, and undo the clips which retain the abs sensor onto the wheel well liners and brake hose. Quite simple once you figure out the clips.
Then with the abs sensor wire hanging off, you can take off the wheel hubs.
They have 3 E14 external torx bolts which hold them on.
You can use the correct size of a normal 3/8 socket to get them off, but I'd strongly suggest picking up an external torx set so you know you can reuse the bolts. Walmart even has a set of OEM tools external torx for a very reasonable price.
When you take out the 3 E14 bolts the hub will likely be seized in place from corrosion, so I leave the top bolt halfway in and hit the hub away from the knuckle with a hammer.
Then installation is reverse.
A few tips would be,
-disconnect the abs wires in the engine bay before taking off the wheels so that it's easier to reach and access
-have a wire brush and brake cleaner handy to clean out the mounting surface that your new hub will install into, as they will be corroded in place.
-use bearing grease around the mounting surface of the new hubs before installing them into the knuckle.
-clean off the brake rotors with brake cleaner and keep them fingerprint free
-good time to add brake grease to the anti rattle clips on the caliper bracket
-pick up new axle nuts for installing the axles back into the new hubs. Many reuse the old ones, but if done, alot of times they will loosen themselves as they actually are meant to slightly deform when tightened, to ensure they stay on.
Also, I'd check before starting the job, that you have a socket big enough to fit the axle nuts..