1994 S10 Blazer Four Wheel Drive Problems

Hi, I have a 1994 S10 and I'm having a problem with the four wheel drive. It has a switch on the dash. You can put it in four high and low both worked, but now it will go into four low but not into four high. On top of that if you drive it in two wheel at 50 or higher it seems to kick into four wheel drive, but then you slow down to 40 or lower it seems to kick back out. Any suggestions on what could be wrong would be appreciated.
Thanks.
Debbie

Mine is 1983. Had to change vac switch on t case once and clear vac lines of t fluid. Found loose vac line connections. Also check if lines plugged. Also at front diff where the actuator pin goes thru diff seal, had tiny leak. Dunno if related but i noticed it did not move easily by hand after i disconnected cable. I rubbed an oily rubber rejuvenating fluid (Wynn's engine stop leak i think) on that shaft and rubber seal, moving it in and out by hand to work in in. It then moved freely and stopped leaking diff fluid there.