Do you know how old the last rebuild is your running ? (miles)
My Syncro did this ~poping out of gear just months before it gave up, needed rebuilt.
Sean G 

    On Wednesday, May 1, 2019, 4:14:04 PM MST, WILLIAM WITZ <pbwitz@SBCGLOBAL.NET> wrote: 
 A few weeks ago on our way home from central Oregon coast about 55 miles from our home the van started to pop out of 4th gear on I-5.I checked fill as best I could. I did not see any signs of leakage but added one Qt. of Amsoil 75 w-140 Severe Gear Synthetic. We drove over to Pinnacles NP and drove home without any further problems from the transmission. I plan to drain the oil and refill it with the same weight Synthetic Oil hoping this will prolong the life. Any wise words from any one?
Bill Witz 84 Westy Tiico