It is a 1996. I did try driving in 3rd or overdrive off and it still happened. It is felt and heard. After the flush my wife said it is worse. Yesterday it did it at a lower speed. Because of cash flow I may have to get a used tranny and pay someone to install it.
It sounds like your wallet got flushed, see it quite often in the dealer I work at. The tcc could be causing it, tap, or ride the brake very lightly at speed. This will disengage tcc due to brake input. I am a bit confused though, may have misread. Is this a vibration felt, or a vibration type noise heard? TCC typically applies at around 45 mph/3rd-4th gear. The shifting in/out of overdrive(4th gear) can be remedied, drive in (d) range 3rd gear and see what happens. The engine may rev a little higher and load qualities change, but if your rumble strip symptom still occurs or not is the answer. You could even be experiencing a misfire. Try duplicating the symptom on a grade(uphill), if this influences it you might look in the secondary ignition system(plugs, wires). Nowhere in the post does it say what year the LeSabre is, its obvious that it is pre-obd II(1996). Had a vehicle(pont. bonneville) several years ago that had two converters put in for a TCC shudder that was just a leaking plug wire. Gerald