The suspension on my 2008 Jeep Wrangler squeaks when I hit a low spot (maybe 4-6" from the grade), even at slow (<10 mph) speeds.
Where can I lubricate or what should I check to eliminate this squeaking?
Claptrap is on the money with grease points, but many modern cars don't have them, and I can't speak to how the Wrangler is setup. That is the perfect starting point, though.
The next step would be to spray down all the rubber mounting points (suspension bushings, shock mounts, etc) with white lithium grease to see if that quiets it down at all. I fear that a normal spray bottle won't be able to get into the 'inside' surfaces of the rubber parts that are causing the squeaks. That leaves you with the final option of unbolting the problem elements so that you can rub some grease on the rubber surfaces. The downside with this solution is that suspension can be very dangerous if you loosen the wrong bolt. In fact, it can be deadly.
So, I would recommend trying the grease points, and the white lithium grease options before proceeding any further. Hopefully, that fixes the issue.
I have three Wranglers, and there are plenty of places on them to squeak, and not all of them have grease nipples.
The most common places that will cause a squeak on the Wrangler, in order of likeliness)
The track bar, drag link, control arms, shocks, springs and sway bar all move when your suspension travels, so they're all suspect. Check the axle ends first, as they are most exposed to water that can wash away grease. For the greasable ones, use your grease gone and grease them, for the others, you'll have to remove the bolt, apply grease to the rubber bushing and reassemble.
U-joints can squeak when they get older, but that usually causes a rotational sound, as will a bad wheel bearing or brake issue.
If it's none of those things, it could also be a simple as a loose bolts in your bumper or somewhere else that's allowing to parts that shouldn't move to rub against with heavy movement, but given it's happening at slow speeds, I'd check those tie rod ends and bushings first.