1988 Jeep YJ strange vibration between 45 and 60 mph

Hello "Car Pros". It started around the 200,000 mile mark. I will be driving between 45 and 60 mph at 2000 to 2500 rpm. If I make a lane change and cross the lane devider bumps, the jeep starts vibrating and it feels like I'm driving on a washboard or a cattle guard. It goes away when I slow down to 40 mph. Then I speed up and keep going. It won't happen at 65 mph or faster. It doesn't happen all the time. I drive an average of 75 miles a day and this vibration happens about once a month. When it first happened, I took it to a Jeep shop and they replaced all the bad looking universal joints. That night it happened again. I took it to a drive line specialist and they rebuilt the rear end and put in new bearings. That didn't fix the problem either. I had it alighned and still no cure. I've been living with it by driving accordingly. Only certain road conditions will set it off. Thanks for any help