bumpy ride?

i have a wierd problem with my 2004 silverado 4.3L V6. every once in a while when im driving on the freeway going around 75+mph my car starts to shake alot kind of like from front to back but really fast as if i was hitting a bunch of little bumps. is there any obvious problem im missing? i thought maybe there's just bad suspension like it's too stiff or something, or maybe should i try adding or releasing some of the air in my tires, maybe just the back or just the front or both. im currently running all 4 tires at about 33psi (last time i checked). thanks for any suggestions.

bump