i have a 1996 f150 with a 302/v8 automatic--it seems that the truck seems to be for lack of better words, straining to go. it has planty of power but seems to be some resistance - let your foot off the gas it slows right down-sitting at light with foot off brake it will not pull itselg forward. it may just be me-dunno?

 

Have you checked your brakes? If they are too tight, it may be like you have your foot on the pedal the whole time. Have you had them changed recently? If so, You can try readjusting themself by turning the adjusting wheel inside the back drum on each side of the truck. If that doesn't work, maybe someone else can help. This is just the place where I would start.
Andy