I have the same issue with the SM420 in my '58 1.5 ton. I have changed the gear oil, the old stuff looked like mud. I used a mixture of Diesel Fuel and ATF to clean the internals. I placed the Truck on jack stands and allowed the Transmission to turn in gear at just an idle with no load. I didn't do this for very long, 5-10 minutes at the most. I left it open to drain overnight and refilled it with 90 wt. mineral oil from Tractor Supply. I did work a lot better, but I suspect, as Kent suggested, that the second gear synchro is really worn.
