[Rocky]

Where’s the protective seal stopping dust dirt etc from getting into the shifter tower?

go get PB Blaster ad dry silicone lube. Give the shifter mechanism a good soaking of PBB.
Then take the truck out for a long drive. You want it to be nice and hot. Pull over and stop. Nograb the PBB again and dose it. Next step is best done from passenger side. Grab the shifter and try to see if the diff lock feature wants to engage.
If the Rover gods are with you it will be looser and may even engage. Then get behind the wheel. Assuming you are on hard surface, with DL off, try to engage low range.
It might free up first time, repeat as necessary.