Had a real wonky problem with the blinkers, but just the right rear.
After chasing the wires through the chassis, looking for a bad ground and finding nothing, I paid a little more attention to the behavior. As I was fiddling with it in the driver chair, I found that by applying and holding a little extra pressure to the blink stalk on the steering column, the right blinkers would activate, staying on as long as I held the pressure on the stalk. Got to be the switch, then.
Disassembling the steering column covers was pretty straightforward and requires the use of Torx bits. The two on the top of the column are T-30, while there is a one T-30 and one T-25 underneath. Once the covers were off, I was able to verify the switch was no longer making contact to turn on the blinker, unless the stalk was held in the “up” position. This is less than ideal if you are using both hands to make a 90-degree turn.
I picked up a new blinker switch assembly from Freightliner NW in Lynnwood, WA for $148 ($162 w/tax). Removing the existing broken switch entailed removing two phillips head screws and unplugging the wiring harness. It's a little awkward working around the steering wheel; you will have to turn it several times to get access to the two screws. Since you're removing the screws, you may want to take the opportunity to replace them with stronger stock. The screws that are provided are weak, maybe made of aluminum.
Big shout out the previous owner who stripped the screw for me. That was an adventure.
Anyway, replacing the switch fixed the problem. All four turn indicators are functioning as designed. Big thanks to Freightliner Northwest for having the part in stock and having a friendly and professional group of people at their Lynwood location.