Headlights acting funky

The headlights on my 1999 Chev K1500 are acting funky. There are four headlights. Normally, I'll turn the low beams on and the two inside headlights come on and when I switch to highbeams all four headlights come on.

Lately, when I switch to high beam, the outside headlights come on and the inside headlights go off! Low beam works fine (the inside headlights come on).

Any ideas? Is it the headlight switch?

It sounds to me like they are working just fine. As far as I know, on all 4 headlight systems, the stock operation has the lowbeams turn off when the high beams go on. You can buy and install a simple little relay to make all 4 turn on when the high beams are on. Have you had the truck since it was new? If not, trace your headlight wires through the engine compartment and look for a few splices and a relay (just a little 1" black cube.) If your truck had this installed and it came undone, the problem you are describing would occur.

In all honesty I think your headlights are backwards. As far as I know ALL four headlight systems work light this, unless they are modified,

2 outside lights -- low beam
2 inside lights -- high beam

Like the previous poster stated there is a way to make them work together.