I would assume it's the fuel filter, too. I had an old Blazer I picked up for less than $500 with the same issue (even at the same speed). All she needed was a new filter (for a while).
And as pelontks stated, the fuel pump would be my next guess, aswell. Learned about that thanks to that crappy old Blazer, too.
Last thing I know about would be to re-tune your carburetor, but that can be a PITA your first time or two, especially depending on what carb you have.