You could put a vacuum gauge on it and rev the motor,to see if you are pulling full vacuum.
You could put a vacuum pump on the actuater, to see if it will lock in and not slip that way.
You could lock the cable with vise grips and drive to see of it stays locked in.
Any off the above will help narrow down your problem.Your slip collar could be messed up from too much grinding,but you wont know untill you manually lock it and drive it hard.
I have blown up more s10 front driveshafts then I can count.When the cv joint goes out they make a huge grinding noise in 4x4,and can fool you by kinda putting power to the front wheels,untill it totally self destructs.check it.
I have owned close to a dozen s10/jimmies and built several with v8s.I know you dont think you have atf in your vacuum lines,but I never worked on an S10 that didn't,unless it was new.It only takes a few drops to deterierate rubber over time.