My trucks got a little shimmy

Hey guys, I was pulling through Pa last week (which either means I am going up hill or going down hill) at about 80k. I was coasting down hills pretty good (maybe 75 or so) then would immediately start back up another hill and a couple times when I hit the throttle the truck would vibrate fairly hard,, I would let off the throttle for a second,, then get back in it, and it would be fine.. I am an inexperienced owner and I haven't dealt with a u joint before, but this sure feels like what I think it would if I had a bad u joint.. only happened a couple times, and always coming off a pretty high speed coast to starting to throttle up heading up the next hill.. Any of you guys ever deal with this? Thanks brothers!