im a noob, and not very mechanically inclined, lol. so here goes.
When I am driving above 65 I lose acceleration, yet my RPM stays up at normal. The truck will then slow to about 40mph and then I can re-shift and accelerate again.
I was told my fuel filter was still good, could it be something else?

 

what year model? check for codes. any modifications to engine?

 

its 1998 4x4 2500. no mods. where would i check for codes?

 

Take it to your local garage, or Auto Zone, and hook up a scan tool to your truck. I hear Auto Zone will do it for free, so maybe try there.

Don't mean this in a bad way, but is your truck a 12 valve 98, or a 24 valve 98.5?

 

if your Rpms are staying up sounds to me like your transmission is slipping but you make it sound like this is a manual shift?

Do you have a CEL on?

 

its a 24 valve, and a manual transmission. Thanks for the other responses too.

 

So the RPMS stay up but the truck slows down? if that is happening on a standard the clutch is slipping!