Well, we got it back in the field tonight.  We also found cracks in another walker (but not plugged).  Walkers are less than 2 years old.  We all think they were probably not timed correctly when they were installed.  Doesn't explain the plugging.  Now they are all in good shape and walkers timed per the Technical Manual.  Fingers crossed that they last this time.  If it happens again might just have to have a combine fire.