Nice job addressing common issues with this era of GM truck. Also, glad to see you getting back on your feet.
As others noted, the brake lines are prone to rupturing due to corrosion. Usually everything is fine and normal then it pops. Most often it is the pressure line leading into the ABS module along the frame rail under the driver. A complete stainless steel brake line kit is about $125 USD. given the extent of rust on the undercarriage I would strongly encourage you to replace the brake lines before they pop.
The OEM receivers were awful in this era. IF you come across a Class IV at a good price I'd suggest replacing it. You TT is small so not a major stress to the receiver, but the combo of poor design and substandard welds created a lot of flex.
The ABS issue can be fixed without replacing the sensors. My '04 2500HD developed an issue where the ABS would kick in under 15MPH as I slowed down and I would lose brake power. The cause was crusty rust building up between the sensor and the hub housing causing the sensor to set too far from the tone ring. I had found a TSB spelling out a procedure to remove the sensor, scrape/wirebrush away the flaky rust, paint the area with rust inhibiting paint, then reinstall the sensor. I did this successfully on the driver side by the passenger side busted apart as I removed the sensor. After doing this the ABS issue went away and never had any other ABS problems.
On the wife's '03 Yukon we had a problem where the ABS pump ran continually. I pulled the fuse for the ABS and later found the control board mounted on top of the ABS unit were prone to failed components. The solution was to install a reman control board.
KJ
