GM-GMC: 1992 GMC sonoma 4x4 , 5 speed manual, clutch failures, gmc sonoma

Question
Bought 3 weeks ago, has drove great. Driving on freeway last night approx.60+ mph. Heard a high pitch humming and I could'nt move into 5th. I slowed down and tried to put into other gears-NONE would allow...no grinding sound. I am only able to drive in 4-low and 4-high on the fly...What happened? Appreciate any wisdom. I was told it has a rebuilt engine with 80,000 on it.

Answer
If I am reading your question correctly, you can only drive your vehicle in 4 wheel drive low or 4 wheel drive high.
If that is correct, something has gone wrong inside the transfer case of your vehicle.  You are likely driving with only the front wheels pulling the vehicle.
I have seen clutch failures during the 1980s in S-10/S15 vehicles but those vehicles would not pull themselves at all or would slip while doing so.
You might check with a knowledgeable transmission shop to see if a failure pattern is showing up in the transfer cases of vehicles like yours.  It might be something as simple as a snap ring or retainer or it might be that the transfer case has suffered serious damage.