I ran into a similar problem with a 2009 lmm/4l85e swap out of an express van to a 03 Hummer H2. The glow plug controller, and TCM communicate with the ECM through the EBCM. I haven't driven it yet as I'm currently in the wiring harness modification stage, and not sure if I,m going to run the ABS, and SRS system with the Hummer control system or the van's system. I temporarily jumped the communication wire's at the EBCM connector and corrected the loss of communication codes. I'm not sure if the LBZ runs the communication network through the ABS module or not, but after I jumped the pins at the connector throttle response was much crisper.
Sam