I have a 2006 Chevy Silverado 4x4 with a 6-inch lift and 35-inch tires. It has been lifted for several years and has been a great all-around truck. However, the other day I was driving on the highway when and all of a sudden the ABS light and parking brake light came on, and the message "Service Brake System" showed up on the instrument cluster. The brakes seem to work just fine, but every time I have started the truck since that day, the message appears. I've checked the fluid level and the ABS fuse, and even disconnected the battery to see if I can reset the system, but this same warning still appears. I don't know what to do next. Help!
You mentioned that the brakes seem to work fine, but more than likely the ABS system on your truck is no longer working, so it's important that you address the situation right away. Don't wait for a panic stop to discover that your truck's brake system isn't up to snuff.
The best thing to do is start with a good scanner and see what codes the system is throwing. Unfortunately most of the inexpensive parts store code scanners won't pull ABS system codes, so you may need to take the truck to a shop with a good scanner in order to read the codes. Without those codes, it's anybody's guess as to what the problem could be.
You've already checked the fuses, which is a good first step. The next would be to inspect the wiring going to the sensors and the control module. Your truck has three wheel speed sensors: one mounted to the hub on each front wheel and one on the tailshaft of the transfer case. There's also a control module attached to the ABS valve assembly. Any one of these four components could be the source of the problem along with the wiring that connects the various components. The front wheel speed sensors are known to be problematic and can get fouled with corrosion, especially if you live in a rainy climate. They can also be damaged when the wheel hubs go bad (another common issue, especially with larger tires).
The ABS control module on your truck was used for several years on many different truck models and is also known to be a little troublesome. As mentioned, the module is attached to the ABS valve assembly, which may be located under the hood or mounted to the frame just under the floor of the driver seat. There are videos out there showing how to open up, inspect, and repair some commonly broken solder joints on the circuit board inside of the module, but unless you're really familiar with soldering electronic components, I wouldn't recommend trying it. Plus, there have been some revisions to that module that address these failures, and your truck should have a module with those revisions. The wheel sensors can run over $100 each, and the module is well over $200, so it's worth paying a shop if necessary to pull the codes and see exactly where the issue lies rather than just throwing expensive parts at the truck and hoping that one of them solves the problem.