Rumble at 45 mph

I have a 1983 AMC Eagle wagon, 258/4.2L motor, auto transmission with just over 100,000 miles on it. At 45 mph the car starts to rumble badly, up hill, down hill, level it doesn't matter. If I drop down to 35 mph it goes away but comes back at 45 mph. If I drop down to 2nd gear (3 speed auto) the rumble goes away and I can take the speed up to 50 or 55 mph, don't want to go faster then 55 in 2nd.