My transmission lost first gear

Check to make sure the safety switches are engaging. One on the clutch, one in the shifter and/or transmission.

hmmm sounds like first gear is ruined inside the trans if your having to put it in second gear to start rolling may want to have your trans checked out by a mechanic

Loss of reverse gear in an automatic gearbox

Broken Reverse selector Band

Brake material on Selector band is Worn out

Selector band needs sever adjustment

Selector bands hydraulic piston seal had failed

Selector not getting oil to work the piston.

Go to a junk yard and obtain a 1966 Ford Ranger transmission. Remove the old unit and do an exchange.

A 1955 Ford would have a Ford-o-matic transmission which has three speeds forward. The transmission starts out in second gear unless you shift into low.
I am not aware of these transmissions having a neutral safety switch.
The 3 speed automatic transmissions that started in low and had a downshift to second under load were first used in the mid 1960s.

try posting the codes, it sounds like a ECM Engine Control Module(brain) problem.

Be sure to put the truck in neutral, while engine is running, while you disengage from 4 wheel drive (low) to 4 wheel drive (high) and then finally to 2 wheel drive. It is critical to shift transmission into neutral when switching out of 4WD Low, as indicated in your owner's manual. Hopefully, this will help to FixYa problem!

If your sure it is acually going into gear than check the auto hubs, they go bad alot. also you can with help from a 2nd person jack the truck up all 4wheels put the truck in 4x4 and drive and see if both drive shafts spin if they do its the hubs if the front one doesnt, its the transfer case

what is the (revolutions per minute) rpm of your engine when the torque converter engages, was the torque converter rebuilt too? what type torque converter is in the ford ie low stall where it engages the gear box at around 900 to 1000 rpm. has there been any power band adjustment by the company that rebuilt your automatic transmission after the insallation.

transfer case needs maintenance like a transmission this means your slipping gears