1995 Sierra 4x4 Problem

[simpson110]

I have a 1995 gmc sierra 1500 with 125k on it. just recently the 4 wheel drive started to act up. i did a search amd i found many topics where the 4x4 wont engage at all but nothing exactly like my problem. what happens is when i put the truck in 4 wheel drive the light on the floor starts to flash and i can hear a clicking noise from under the truck(only when moving). sometimes when im stopped it will engage, but as soon as i hit the gas it pops right back out. it does this with both low and high. i read multiple topics where the 4x4 wont engage at all and it is the actuator, but before i buy an actuar i want to know whats wrong. well, any advice will be great, and thanks in advance.