Since everything else has checked out to be with in specs or has been replaced with new, the secondary looks like the problem. Less than 3ohms doesn't seem like much, but anything outside the specs is bad.

You might check your local wrecking yard for a good one, before plunking down the $$ at the dealer for a new one.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed