your emergency flasher is separate from your turn signal flasher. I would start by replaceing the turn signal flasher with a new heavy duty unit.

Then I would crawl under the truck and follow the wiring, you may have a bad ground in the back.

The reverse lights may be the ground, or they may be a bad reverse switch.

Also, if you have a bulb out the emergency flasher will still flash normally, however the turn signal flasher is not getting enough load to flash.

The problems we face today can not be addressed at the same level of intelligence we were at when we created them - Albert Einstein Or with the same level of $ - Me