The Stovebolt Site Forums The Parking Lot Big Bolts fuller 10 speed transmission

i have a '72 GMC 7500 with a fuller 10 speed and it doesn't like shifting into any gear when i try to pick up speed on the road, but when i don't try to pick up speed, just shift it's fine what would the problem be. the truck only has 15 to 20K miles on it.