Physically poping the shifter out of 1st gear on its own is caused by worn bearings and syncros inside the tranny causing the shafts that the gears/syncros run on to move when under load putting pressure on the synchro to pop out of position, chances are someone didn't change syncros as they are difficult to diagnose by looking at them and very expensive. Lots of things can cause this to happen, this can happen over time due to wear, fluid maintenance or lack there of, also if the clutch is not adjusted and bled correctly and you keep forcing shifts, or the pedal mech is worn out bent or broken, bent clutch fork, sticky throwout bearing, broken clutch spring, and forcing it in and out of gear will also cause more issues to quickly arise if any of the later items are not addressed in a timely manner. looks like you digging into the wrong area IMO. So it sounds like you have 2 issues, worn 1st gear synchro or bearings, and improperly operating clutch mechanism as it should go into 2nd gear without force. Also list the truck year, package and model, perhaps others have a similar experience with this same set up, my 4R is Auto so no direct experience., but I have rebuilt a few AWD Mitsu Manual trans with success