I was havin the same issue, could never get it to drop down into 2nd or 1st when slowing down.
In my gearbox only 3rd & 4th have synchronizers, so you have to get the truck slowed down enough to get it to drop into 2nd & 1st, double clutching helps alot too (even MORE fun when you add shifting the 2-speed rear end!). The shifting instructions say to double clutch it as well. I've had to practice, but have gotten it down pretty well.

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P