I don't know the exact cars well enough to say for sure, but cars of that era here didn't have other modules, I think the Caddilac did.

Here's another issue pointing towards a code or hardware issue. Look at bold print! Are 85-86 cars having the issue? 87-98 cars or ECM not having issue?

DATA STREAM A28 SPECIFICATION

ENGINE USAGE:
5.0L PFI CPC - (LB9) - (F) - 87,88 (160 & 8192 baud)
5.0L PFI CPC - (LB9) - (F) - 86 (160 baud only)
5.7L PFI CPC - (L98) - (8) - 87,88 (160 & 8192 baud)
5.7L PFI CPC - (L98) - (8) - 85,86 (160 baud only)

DATA PIN : READ DATA ON PIN "E" OF ALDL CONNECTOR

BAUD RATE : 8192

MODES : DATA STREAM ATTACHED FOR ALDL MODE 1

THESE ENGINES USE A 'HYBRID' ECM, AND THE 8192 DATA
COMMUNICATIONS OCCUR ON ALDL PIN 'E'
Serial data shall be unidirectional from the ECM to a connected
device at 160 baud when the ECM is not in the ALDL mode. When the
ECM is in the ALDL mode (10k), the ECM shall transmit the normal
25 parameter ALDL list at the 160 baud rate. The ECM shall then
be moded to listen for a period of 50 ms for an inbound message
from an ALDL test device at an 8192 baud rate. If a valid message
is received, communications shall continue at an 8192 baud rate
until a power down reset. Otherwise, the ECM shall be moded for
160 baud, transmit the 25 parameter ALDL list and then listen for
50 ms for an 8192 baud inbound message.
------------------------------------------------------------