'96 Geo Tracker 4-Wheel Lockout
 

I bought a 1996 Geo Tracker recently, and I've got a question for those who are familiar with the vehicle. We just got a fresh round of snow and I decided to try the 4-wheel drive on my way to work. It has manual lock front wheels, so I locked them, then started pulling out of the driveway in 4-high (reverse). As soon as I let the clutch all the way out, there was a distinct clunk, so I stopped and decided that just two wheels would be fine today.

Has anyone else encountered this issue? When I drove the car with the former owner, the 4WD felt fine, and shifting into 4-high from 2 was clean. Could it be because I was going in reverse this time? Or perhaps something to do with temperature or ice/snow? Or am I forgetting something about how a manual 4WD works? Any help would be appreciated.