Started with a 1996 1500 gmc 5.0 5speed
found a 1997 2500 gmc with a 5.7 4l80e with 60k on motor and trans.
this is what i did.
I swapped whole drive train.
I swapped full wire harness engine bay
i swapped full exhaust system including the dual cats.
And interior.
I swapped steering column .
I pulled both dashes and swapped all components including gauges.
I kept all of the wiring harness connected to the drive train when I pulled engine and trans together.
I installed drivetrain in the 1500 connected the interior and enginebay wiring harness.
all grounds have been checked.

Turned key on and have back feed coming thro my stereo speakers. The tone of the feedback changes as i push in the brake pedal.
If i unhook brakeswitch feed back stops but the gauges are still acting the same.
The gauges jump as i push the brake pedal as well.
THE 4wd indicator light is illuminated and the t case is in 2wd.
the engine runs great the trans shifts PRNDL with no problems.

Please can someone please help me I've spent days checking plugs and grounds to no change.
And I should also say everything worked fine when engine was pulled out of doner truck