Oldsmobile/Buick Repair: 4 x 4 GMC Sonoma - 2002, gmc sonoma, torque load

Question
I had my truck in 4 Hi and couldn't get it to go back to 2 HI.  The only way I could get it to shift out of 4 x 4 was to put it in neutral.  Now I can't get it to go back to 4 x 4.  

Can you please email any advice you may have

Thanks

Answer
Are the tires worn about equally, front and rear, or are the rear tires considerably different wear-wise than the front?
If they are worn unequally, there could be a torque load building in the transfer case, causing the hard shift out of 4X4.
Try coasting very slowly as you try to engage it, or disengage it, maybe changing from foreward to reverse, and back.

Van