when i get up to about 70km/h it kicks in and out of 4x4 i have a 1998 blazer with electronic 4x4 without the auto 4x4 button any ideas why

 

there is an atc fues under your hood! if you pull it, it should stop that from happening! lol. it the automatic transfer case unit! it 3 bolts and its up under your transfer case! that is what needs to be replaced! i think?

 

Try searching "vacuum actuator".... I have the same problem with my 98 4x4 and I'm pretty sure that's the fix you need.