4X4 taking long time to shift

96 S-10 4X4

When I push the button for 4wheel drive, the light lights, I can hear the actuator shift but I won't go into 4wheel drive. I can sometimes go for a mile when I will get a sudden jerk and then I will be in 4X4. Time to shifting is variable. Anywhere from a few seconds to minutes. I have had it to the dealer 4 times and of course it behaves perfectly for them. No error codes are set. Where should I be looking? My impression is thats it's RPM related as I always have to race the engine to get it to shift if I'm standing still. Vacuum?

I don't know if the push button has vaccum, but if it does that's what I'd look at first.

Yeah, that '96 used vacuum to engage the front diff. Check the lines and that switch connected to the transfer case, they tend to go over time. Also, make sure to check the vacuum actuator itself for leaks.