Transmission quit working suddenly

[MidwestAl]

I gave my old 1994 4.0, 4wd, automatic trans. Explorer to my son who lives a couple of hours from me. Recently, while driving normally on the highway; the transmission stopped working completely. No weird noises, no sudden loss of fluid, it just stopped working. The only problems I've had with automatic transmissions seemed to have some warning signs; noises, leaks, etc. Any ideas?
Thanks in advance!!

 

[BrooklynBay]

Check the pressure with a 0-300 PSI gauge on the rear port. # 10 in my list of useful threads has more information on how to do this test.