2003 Silverado weird turn signal issue

Very long story; bear with me please...

Quick back story; lights supposedly worked fine a few months ago (before I
bought it from a friend).  Truck had a body lift installed and I paid someone to
remove it.  The guy that removed it obviously didn't know what he was doing.  
Bed was also removed from the truck in the process and then put back on.

In the process of removing the body lift, he didn't re-connect the 2, large
grounds under the rear of the cab (one on each side), but I put those back
on.  Everything else appears fine.

With the headlights off, all signals work fine.  When headlights are on:
- ALL rear lights blink (tail and turn flash alternately) no matter which signal is
applied
- CHMSL & plate lamps flash too
- when brakes applied, lights quit blinking
- with cruise control on (at highway speed), cruise will disengage when
signals are applied.

Again, these problems only happen when the headlights are on.  I've double-
checked, cleaned and tightened all grounds I can find: 1 in rear under the
bed (mounted to the plate above spare tire), 2 large grounds under the rear
of the cab (1 on each side), 2 small grounds on frame bracket in front of the
driver's door, 1 near the front on the driver's side and 1 under the hood that
goes to the back of the engine block.

I've replaced the turn signal switch and flasher relay with no change.

I should also mention that when the body lift was removed, he forgot to
remove the ~2" spacer that was installed on the steering column shaft that's
inside the engine compartment (it's since been removed).  I thought this
might have caused the column to bind somewhat and maybe pinch
something in there, but I removed all the cover panels in the cab in that area
and can't find anything visually wrong.

I'm at my wit's end here...any suggestions?  I've got videos of the problem
occurring, but I don't know if they'll post here (YouTube).

-WRM