Edis, I find mine goes into 4X4 quite easy at any speed, even at 40 + mph. Of course it can't be put into first while moving. I DO find it difficult to take it out of gear at speed or on pavement.
John, it sounds as if you have the reverse idler backwards maybe. I can't remember whether there is a difference between fore and aft but if there is, then that will be your problem. It's trying to hit first and reverse at the same time. To test this, try not putting the gear shift all the way into the first position. Try holding it back a bit and see it it still locks up.
Bill H.Welfare, a modern day career choice.A gun is like a parachute, if you ever need one and don't have one, you'll never need one again1962 AMC M422A1
1965 Stevens M416B1
My 1944 Willys MB1944 Willys MB