can't engage 4WD

Hi, I recently had the privlage to inherit my Dads showroom condition 1991 trooper, its a fine truck. I has hardly been used in the past 2 years and during the recent snow I followed the instructions and the green light on the dash indicates that the 4WD is on, yet I feel it is still in 2WD. When I push the winter mode button, it lights for 30 seconds then goes out. This truck has auto-hubs. Can anyone advise? Thanks Kathy