The problem could be anywhere in a vacuum system that stretches from the intake manifold to the rear axle.
The easiest way to find the problem is to start at the rear and work forward.
Take the 2 hoses off the vacuum actuator on the diff, (mark one of the hoses so you know how they go back). Engine off, trans in neutral.
Use an air hose with a rubber nozzle to give a squirt of air into one inlet on the actuator. Listen for a clunk when the gear moves. If nothing, try the other side. Listen for air rushing through a bad diaphragm.
Even if the diaphragm is bad, a rush of compressed air can often overcome it to make the gear move.
When you hear it shift, stick a couple bolts in the hoses in case they still draw vacuum and try driving it. See if the speed changes.
If the air won't do it, take the cover off the actuator (lots of #10 screws and nuts) and push or pull on the rod to shift the gear.
Hint - 1st gear low range is equal to a fast walk at full throttle.
