'92 Sport issue with 4th gear overdrive.

[Good Sport]

Hey , it's been awhile! I'm approaching 30 years ownership of my '92 Chevrolet Sport (reg cab/swb) that I purchased new in 1992. It just turned 158,000 miles.

Unfortunately, it has developed a problem during highway speeds around 50-60 mph. It begins to shudder or buck while the overdrive has engaged (shifts fine into overdrive though). If I let off the gas it will smooth out and return to normal. There is no issue driving in 3rd, or other gears. All my sensors etc on top of the engine are new or recent.

With the above description, does anyone here have a clue what I need to check? Thanks.