Although there is a reasonable chance that the egr tube is blocked,[common
104 prob], an easy test for vac. flow is to simply apply some vac [ hand pump or vac source] to the egr while at idle. This opens the egr and will cause the engine to stumble..[ go lean]...if it does ,the tube is not blocked and other parts of the egr system are suspect...
You may know about the EGR/TSB on this chassis...[ archieves]