My 1999 navagator is stuck 8n 4 wheel drive low. Any suggestions on how to fix this?

 

Try going in the opposite direction of whichever way you when last before shifting out. Sometimes driveline windup may possibly prevent it from shifting.

 

if your in a cold climate it may be frozen , try parking in a warm garage over night.
I have had this happen on a couple ford trucks

 

Did you try what I posted in the other thread you started on this subject?

Set the parking brake, chock the wheels, start the engine, shift to neutral, move the selector to 4hi or a4wd, slide under the truck and tap on the shift motor with a hammer. If that doesn't jar it, then you can remove the shift motor and move the cam by hand to get it back into 4hi, plus you'll have to replace the shift motor.

You can also do this with the engine off, key on, if you feel safer that way, but with the engine running you'll have about 10% more power for the motor to kick into 4hi.