hi , every once in awhile mostly enter a highway from another highway i wil be at about 50 -55 and get on the thottle to increase speed the truck shudders , tach does not increase , is this the converter , or trans slipping .

 

If not engine related, sounds like a clutch pack in the tranny getting worn, or low line pressure causing one of the servos to not be fully engaged and therefore the clutch stick-slips. But it could be lots of other things.

Check the fluid level??