When you got a new steering box and pump, did you get the ones specific to your truck with variable steering?
Also you should also try changing the EVO sensor (as some people call it) under you dash on the steering column. Its a $30-$40 part depending on where you go. There is a valve (EVO, electronically-variable orifice) in the PS pump which adjusts the amount of PS pressure applied to the steering components based on the information provided by the steering wheel position sensor. Apparently the sensor gets old and starts sending erroneous signals out which ends up commanding sudden spikes in PS assist from the pump, thus causing a "jerk" in the steering wheel.
GM Part number: 26064468
Instructions:
Its the plastic looking thing in the pic.
I got these instructions from another site:
This job takes about 15 minute.
Tools Needed:
15mm box wrench or socket & ratchet handle, small flat screwdriver, flashlight or trouble light.
1. Find the bottom of the steering column inside the truck, it's about 6" or so aft of the firewall. Ahead of the column, you'll see the steering shaft with a bolt and nut through a clamp.
2. Start the truck, and turn the wheel so that the nut points to about 5 o'clock.
3. Stop the truck, and disconnect the negative battery cable.
4. With the wrench or socket, loosen and remove the nut and bolt from the steering shaft clamp.
5. Slide the shaft forward far enough to get about an inch or so of free space at the end of the shaft.
6. Looking at the bottom of the column, you'll see a plastic ring about 1/4" thick. It could be black, tan, or blue in color. That's the EVO sensor.
7. Move your hand up to the top of the column, you'll feel the EVO wire harness as it runs along the top of the column. Move back about 4 inches or so until you feel the connector. The harness and connector are wrapped in foam tape, so pull enough of it away until you can see both sides of the connector. With your small screwdriver, gently move the connector lock until you can unmate the connector by pulling on the sensor side of the harness.
8. Pull the sensor from the bottom of the column. You might need to use your screwdriver in the seam between the sensor and steering column to get it started.
9. Replacement is the reverse (of course) of removal. When you replace the sensor, note that it is keyed and has to enter the column with the wiring harness on top.
