[Alex V.]

Agree with everything said above. From the sound of it your transmission has an unsynchronized 1st gear (i.e., it isn't designed to be shifted into 1st while the truck is moving - pretty much a standing-start or uphill-takeoff pulling gear). (Note: This type of transmission is commonly referred to as a "granny low" 4-speed.) I know on my Suburban's tranny, the gap between 2nd and 3rd is much wider than between 3rd and 4th so it's necessary to stay in second longer than what feels natural to make the shift. I don't judge my shift points by RPM and never have, so just drive it and get a feel for the comfortable shift points; you don't have to keep it wound like a 3-day clock, but if the engine is lugging (RPM's are too low to generate any power and the engine struggles to accelerate) after letting the clutch out just move your shift speed up a bit. It should come rather quickly and after that it'll stay with you from now on.

Good luck and have fun with it!