Because the counter shaft (cluster) gear is out of alignment .
You'd best take it apart A.S.A.P. and see what's what, quite often the counter shaft proper is badly worn .
I saw a '37 once that they just kept driving it low on oil and the cluster gear wore about 1/4" (
!) into the case ruining it .
Your truck has a *very* light duty cluster gear, it has a bronze bushing in it and this wearers .
Also the cluster gear needs to be shimmed as it travels back and forth as you load and unload the gearbox ~ this is why it pops out on decel .
The _GOOD_ news is : this gear we re designed my GM for the 1941 year model so it has proper needle typ roller bearings that fit right into your earlier tranny .
Best bet is to find a takeout tranny from a Hot Rodder ~ they're worthless and so usually cheap or free so GET ONE and read it, lots of good info and you'll
see how simple and easy it is .
Yes,
YOU at home can overhaul this simple gearbox with simple tools .
The four speed version you're wondering about has a compound low first gear that only moves the vehicle at walking speeds ~ quite handy on the farm amd in commercial service .
It has a flimsy little stamped sheet metal lever on the right side of the gearchange lever, this lifts up a thin rod going into the gearbox and disables the reverse lockout .
The trannies up through the late 1940's are generally interchangeable ~ all you needs do is count the input and output slines and you're golden .
I forget where you're located but I still see these trannies in Auto Jumbles .
Before you pay for another one,
COUNT THE TEETH ! .
It's possible and likely that passenger car cluster gear and shaft will fit .
I
ass-U-me you have a shop manual ? .
If not they're dirt cheap new and often free or close to when used and oily, the information therein doesn't change .
If you decide t buy another tranny, new. used or "rebuilt" (?) Never,
EVER let the old tranny out of your sight until the job is done and you're 1.000% happy .
Many have failed to heed this simple advice, every one who did regrets it forevermore .
(it's about time my useless decades out of date experience comes in handy)