Charlie,
I've been studying the shift rail & interlock functions on the Dana 20. One rail & shift fork control the rear output. The other rail & fork control the front output. The service manual lists a bent shift fork as a possible cause of difficulty shifting into 4WD, but I don't think that's your problem. If the front output shift fork was bent, it should still go into either 4-HI or 4-LO. The manual suggests greasing the shifter pivot pit. I don't see that solving your problem, but it can't hurt. There's a grease zerk on the end of the pivot pin.
If that truck has been sitting for a long time, or if it's been driven but not shifted out of 2wd, I'm thinking that the interlock pins are stuck in their bores. Neither of the pins has to move to shift between 2-HI and neutral, but one has to move for 4-HI and the other has to move for 4-LO.
I'd lock the hubs and drive down the road at 15-20 mph while trying to pull the shifter into 4-HI. Even better is have a co-pilot try to pull the shifter. They can get pretty stiff if not used for a while and they're easier to pull from the passenger's seat.
You don't want to have to dissasemble the shift rods and forks. The set screws come out way easier than they go back in.
Once it's freed up, it should be easy to shift in and out of 4-HI while moving as long as the hubs are locked. It won't shift on the fly easily with the hubs in AUTO because the front axle isn't rotating in that case. With the hubs locked, I've have no trouble shifting smoothly at highway speeds.
Jerry