mandrew,
Whew...you have been around the block on this thing! I'm not sure I understand all I read, but it sounds like you know what you are doing. The detent springs could be broken, but I am doubtful about dirt or crud holding anything from meshing. The spinning gears and washing action of the gear lube usually keep the moving parts clean.
The only spring in the shift box should be the one that holds the lever 'down', or on the 2nd/HI shift arm. To engage 1st/REV, you have to pull 'up' through the gate. Is this right.
If the shifter box is dumb, and the positioning of all linkage components is achieved with detents in the transmission sidecover, isn't this what everything has to be 'zeroed' to.
With the shift rods off, can you move 2nd/HI lever to a crisp, positive detent where 2nd would be. Unless the fork is badly worn and not shoving the synchronizer far enough engaged, the detent would be the correct position.
Broken, loose or oil weakened motor mounts will allow the motor to torque and play strange games with the shift linkage. I'm reaching for straws, but I would take the truck out in front of the house, drive it until you were in 2nd gear. Shut it down and without moving anything, pull the shift rod for 2nd off the transmission shift arm and see if that shift arm was seated in the detent.
We might not be on the same page on the problem. Keep us posted.
Stuart
