I have a moen shower control that drip leaks when off. It's a pull knob for on/off, rotate for hot/cold. Its an all chrome unit. I need to get into the wall, but I cannot figure out how to get the knob off. There is no set screw, and I've firmly messed with it, but cannot figure it out... any help out there?.

i cannot figure out how to get the knob off.
The insert on the knob pops off, (see image) and you unscrew the screw,
Shut the water off and open up the valve and replace the cartridge, Clicking on;http://www.moen.com/consumersupport/installation/moen0133/moen0133.cfm
Will get you to a installation guide for replacing a Moen cartridge. You can get a new cartridge from Moen for free call them at 800-289-6636, While you have the valve open turn the water back on for a moment to flush out the supplies. Rerassemble and test. Good luck, Tom