Spindle bearings/bushings are the likely culprit. Don't replace them though; try lubing them first and that'll prolly take care of it. Here's a cross-post from yotatech (http://yotatech.com/f116/spindle-lubricator-ifs-axle-shaft-pictures-100327/index2.html).

"If anyone has avoided doing this due to not having a spare axle nut (like me), I found a solution that may be easier.

Basically, I took a 1.5" pvc pipe cap, drilled a hole in the end of it, and bolted it to the end of the axle shaft (after removing the hub of course). I drilled and tapped a hole for a grease nipple, then installed that in the pvc as well. I cut some gaskets out of an inner tube to seal the whole thing. It worked very well, and you don't have to remove the axle nut. Also, the axle shaft is held in place so it won't be difficult to get it back in when the whole deal is full of grease.

Here are some photos:

First, pull the hub. Then remove the retainer circlip and washer:



Then, bolt your simple little pvc cap onto the axle shaft, with a couple homemade gaskets. I got a longer bolt to use, but then had to stick some other washers and a nut on there as spacers. I've heard a M8x35 bolt will fit perfect though:



Before tightening the assembly and pumping it up with grease, make sure to push the axle a little ways in and stick a spacer back there so you can see when the grease starts to come out (I stuck a 1/2" extension in as a spacer). Also, you could probably shorten the pvc cap by 5/8" or so and waste less grease in the process. Here is the back side, showing the grease starting to squeeze out:



Even though I had to buy the 1/4-28 tap for the grease nipple, the whole thing cost 10 bucks or so, and was really really easy to make. Also, as I mentioned above, you don't have to remove and re-set your axle nut, and the axle won't get pushed out during the process.

I did both my axles two winters ago if I remember correctly, and have had zero noises or problems since then. My truck has 270k miles on it, so if this worked that well on mine, it should work on most (if the bushings aren't completely destroyed of course).