On another forum there was a discussion about this problem on the Workhorse chassis. I wouldn't say it was a GM problem but I would say it is a Chevrolet problem. I have a 98 Yukon and an 04 Yukon and haven't had any problems with the wipers.
I did have the same problem with the wipers on my 88 Silverado and on my 06 Allegro Open Road on a Workhorse chassis. After taking them in to the dealer and having the same results you seem to be having I made my own modification.
I pulled the fuse under the dash and replaced it with with 2 wires with plugs plugged into where the fuse normally plugged in. Make sure the wires do not and can not make contact with each other. I then put an inline fuse in one of the wires and hooked them to an on/off switch which I mounted on the dash. This way when the wipers won't turn off I can flip the switch without having to get out and disconnect the plug. I can still use the normal switch to control them and the washer.