You have the main trans that is a four speed, then there is a large round casting mounted to the rear of the trans that is the high/low clutch. It has its own valve body and shift rod to it at the top. Check that the linkages are not rusted, preventing shifts. I had high/low go out one time. There is a large cone clutch in the round casting that does the shifting. It has a large seal ring around the OD that had failed because of a broken flange on the cone that retains the seal. I took off the cover on ther round casting and replaced the cone and seals, fixing the problem. These trans are tough, but do not continue to run engine oil in it like the military recommends. Go to Dexron ATF for long life. Engine oil is all wrong for them, and is the biggest contributor to the trans having a bad name. I did that on mine, and I used it for 20 years as a grain truck and plowing snow, also pulling a 35 foot machinery trailer with no trans problems. Also under the trans at the front is an oval cover. Drain the coolant out of the plug under it marked water, remove the cover, and check for accumulated crud that needs washed out. This is the trans cooler, and sludge in it makes the trans run hot and leads to failure.
Regards Marti