That definitely sounds like a bad driver's side retarder, but you might want to try having them "re-learned" and see if the code stays away for awhile. That can work if the problem is a retarder getting weak or losing too much clutch material, the re-learn will allow the ECM to compensate by driving it harder electrically. If that doesn't help then the left one is probably worn out, but replacing it isn't a big deal, especially the driver's side. Motor quotes 1.1 hours. The righthand side is more like 3.5 because of all the crap in the way, but it still isn't major surgery.
You'll find stuff out on the net about the drive plates getting scored or having runout (that's the disk on the end of the exhaust cam that acts like a phaser), but I'm assuming your engine has had decent maintenance. If so, chances are it's just a weak or worn retarder and the drive plate is fine. It sounds like the shop that quoted you was planning on replacing the plate, but unless your engine is a pot of sludge, that's almost certainly unnecessary.