Truck Repair: Auto trans. problem, differential assembly, vacuum leak

Question
 Hello Van,
 I have a question my son related to me about his recently purchased 1995 chev.2500,4x4.  Seems the tranny shifts o.k. in 2 wheel drive except it doesn't kick into passing gear when entering a highway,and it will hardly move forward or backwards in 4 wheel drive (transfer case on floor).
Do you have any thoughts about this?  Hope i explained this o.k.

Thanks,
Dick  

Answer
Hi Dick,
The foreward or backward problem may be due to the front axle actuator not engaging, and the transfer case differential assembly shifts all the drive to the front driveshaft, which is just spinning.
Have him get someone down there to see if the front driveshaft isn't turning at that time, even though the truck isn't moving.
95 may well have electric shift, so I don't think a vacuum leak would keep it from passing gear, but look for a vacuum leak anyway.
Van