From what you said l believe this is a cascade sideshifter you will have to take it apart again under the metal channel in the center there should be a small about 1/4inch square block welded to the metal channel some times it brakes off it sits in the center of the carriage that what stops it from moving off center also you said that you greased the metal channel there should be two plastic wear strips that sit on top of the metal channel that is how the sideshifter slides it should never be on the metal also there should be two plastic wear blocks on the back of the carriage that is how it is built if there are not there they probably fell out when it went off center you will have to get them and put them back as it is possible the sideshifter could fall off and someone could get hurt. There should be a stamped serial number in the side of the sideshift carriage use that number to get parts hope l was of help.