When I try to use the left blinker, the right one comes on

You obviously have a bad connection somewhere in the taillight. Some trucks it is a common problem and the taillight circuit board is the problem.

The problem was a bad left front blinker socket that was grounding out

I would suspect the turn signal switch is defective. Do not place any parts until an qualified GM tech has confirmed this guess.

check for bad circit board or for a bad ground on that light and circit board.

either check for voltage at the blinker and also check for left blinker fuse . NO voltage at connector there is an open in wire replace try to find the open torn insulator or wire being exposed.

The right blinker issue sounds like you have one bulb out either front or rear check them both. The left blinker issue sounds like either bulbs(both out) or fuse, or worst case blinker switch has gone bad on left turn only so not picking up the signal.

Sounds like wrong bulbs in some sockets

Flasher relays relies on bulb wattage for the speed of the blink.
You probably just have a blown bulb in the left rear blinker.:)

the rapid blink is because there is a bulb outage some where, Pull Over turn on left Blinker Get out of your car look at the front left blinker hen walk to the rear of your car and look at the left rear Blinker light then do the same for the right side blinkers if all are working rapidly your left right switch is bad

RIGHT BLINKER dead ,front,back, or both..???? If front works, and rear doesn"t (REPLACE REAR BULB).....and vice a versa....If rear and front both don't work replace the FLASHER ..Flasher is located in the FUSE block area.......Listen to the operation of the LEFT or RIGHT flasher operation and you should notice a dirrerence....