Yeah, switch the truck off, put the transmission in neutral and see if it will go into 4Low.
If it will, and will work there, then its your shifting method.
I have one sitting out in my barn I need to attack. I ordered my new truck from the factory with BW1465 with a PTO opening.
When it came in, they had discontinued the PTO. So, I bought a rebuilt 205 and installed it.
Since that was not an option for that truck, they had mounted a high pressure fuel pump on the frame where the support for a 205 used to mount.
I designed up a rubber supported mount that bolted to the frame and mounted to the three holes on the side of the case.
Then, about 150K later, I started noticing a vibration when I was in 4wd. Turned out the front yoke nut had backed off and the yoke had slid out enough to let mud get in the seal and damage the front output bearing.
When I drained the oil, it was black with very fine metal dust.
I needed the truck, so I bought another rebuilt one, and put that one up for rebuilding.
But, the second one, the hole pattern in the case did not match up with my mount. So, the only thing holding that heavy cast iron transfer case, plus the PTO adapter and a big honking hydraulic pump is the adapter of the transmission..............
I often listen for the entire back half of my drivetrain to fall out in the road when I hit a big bump. Worked so far, about another 150K, but I really need to drag that old one out and rebuild and install it so my mount will work again.