a stuck lifter, or push rod is usualy caused by contamination. you need to remove the stuck push rod and clean or replace it. a fairly easy job on the 4.3 since you don't have to remove the rocker arm assm. just lossen the rocker arm over the stuck push rod, turn it to the side and pull it out.
complete instructions here:
http://www.autozone.com/servlet/UiBr...3d8012a658.jsp
scroll down to the bottom for the 4.3 engine