Classic/Antique Car Repair: 1977 GMC 4x4 th350 tranny, gmc 4x4, reverse gears

Question
when i 1st start the truck, it will run in all gears, when it gets warm/hot i only have low and reverse gears, im thinking torque converter,

Answer
It has been my experience that the torque converter has very little effect on the shift points of a transmission or whether it will or will not shift. It sound like the warming up of the transmission might be effecting a transmission valve body valve, the governor, or softening a seal in one of the pistons. But the problem sounds internal.
Brad