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Lever is probably not pushing down far enough. There's a detent/lockout mechanism on the linkage, you can see it in action if you slide under the truck and have an assistant work the lever. The up-down action of the linkage is known to get hung up from lack of lubrication; some guys have found a way to drill into the side of the linkage and install a grease fitting.
"Nuclear option" - locate the pin that interfaces with the detent/lockout (you'll see when you get under there, with someone working the lever). Saw that pin off, and no more detent/lockout. You can basically swing from 4HI to Neutral to 4LO without any push-down action. This is how the earlier trucks (up to '86) worked. 'Course, the lockout is there to keep you from inadevertently slipping it into 4LO while underway, so use due caution if you elect this option.
mine old truck wouldn't shift into 4Lo worth a darn. my "new" one with twice the miles is much better.
on the old one, I had to be in neutral, on level ground with my foot off the brake and the e-brake off. even then it took a few tries, but it would go.