Auto trans. 4-3 downshift problem

1992 XLT ex-cab 4.0 automatic w/overdrive
156,000 miles
Regular service on transmission every 30k miles

Transmission drives out great except when going up a grade, it will not downshift from 4th gear; it only makes a coughing (or kicking) sound in the transmission

Any help will be appreciated
Thanks in advance

might as well rebuild trans cause you gotta take it all apart to replace the bad part.there is a overdrive module or a valve depending on what trans you have that goes bad.could also be chatter marks on convertor shaft from low RPM's in OD