What could be causing this popping noise?

There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.

Also look at the steering shaft. It is a two-piece unit to allow it to collapse in a crash. Grab it under the brake master cylinder and tug it up and down to see if there's some knocking there.

Two models of the Chassis Ear. The original one costs $199.99 from the guys on the tool trucks like Mac, Matco, and Snapon. You can find the same thing on Amazon. Com for less than 70 bucks. The newer model lists for 100 bucks more. Four of the six microphones are wireless so you can attach them to rotating parts, and you don't have to carefully route wires to inside the vehicle.

If any weight was placed on the new bearings before the axle nuts were torqued to spec. That would damage them and make them noisy. People often set the vehicle down on the tire to hold the axle from turning when they tighten those nuts and that will do it. I stick a screwdriver in the vent slots of the rotor to hold the shaft while I tighten the nuts. The torque spec. Is critical too. Over and under-tightening the nuts will ruin the bearings.

When does that popping occur and how often? Once per wheel revolution? Over bumps? When turning the steering to one side or when returning to center?

Do you notice the steering wheel change position when this happens, meaning it is no longer centered when driving straight ahead? If something is moving related to a lower control arm that will change the relationship between the spindle and steering linkage causing a change in steering wheel position. As I recall, if you were to stand in front of the truck and look back at the steering linkage, you'll see it is the same height off the road as the lower ball joint. That means a moving lower control arm will have a big affect on steering wheel position. The top control arm, however, can move and have no affect on steering wheel position other than causing a pull to one side that you have to counteract with the steering wheel. You might want to look at the two bolts that hold the upper control arms to the frame to see if they are letting the control arm move back and forth. Those are how camber and caster are set during an alignment. Also look at the control arm bushings.

Don't discount that steering shaft I mentioned earlier. They cause a clunk when they're worn and a pop when they're bound up. They need to slide freely as the body flexes. They typically are noticed on bumpy roads and driving slowly in parking lots. Simply turning the steering doesn't do it.