1994 80. Stock grocery getter with normally worn tires. No lockers. Recently had front bearings greased and replaced. Went on a hunting trip in which I had to use low gear quite a bit and fjord a river twice a day for 7 days (pushing water with bumper) had to go through a nasty muddy swamp hole a few times at a high velocity to get through. Since this little trip a month ago now when I get up around 60 on the highway and let off the gas it sounds like I am driving over a highway rumble strip. Only does it at that higher speed. Doesn't matter if I am in 4th or 3rd gear so doesn't appear to be tranny related. I had fluids swapped in everything btw in case water got in anywhere. Ideas where to start looking for the problem or what the most likely culprit would be? Could it be as simple as a wheel weight being thrown off?