Need Help with my Blazer

I have 95 blazer 4x4 4.3L and my problem is when i am drive only on the high way the blazer revs high constanly, once i go over 80 to 100 it rev out of control making it hard to drive. So i tried driving it on the other D gear( the solid D) but now it give little revs but now the speed gague will say i am going 50 but actually i am going 100, I took it a trany place and they scan it and it gave back the Speed sencor code and also maybe the O2 sencor code.
So tommrow i am changing the speed sencor but I think its the trany it self because it drive perfect under 80 but also it takes off in second or even 3 gear some times.
I don't know what is wrong so if some one can help plz leave some info

Welcome Aboard!

One problem is that if the VSS is not providing the correct signal, the PCM may not shift the transmission correctly. With no VSS input, the PCM may be operating the trans in default mode, starting in 2nd range. The '94 & '95 ECM/M is a little strange, and I've never hacked those, but if the control scheme is similar to other GMs, it needs a VSS signal to operate properly.

80 KPH is not that fast, and the trans may not be in lockup at that speed, especially if there is no VSS signal and the ECM is shifting solely on engine RPM and trans pressure.

Solve the VSS problem first, since it is a known issue, then diagnose from there.

I fix the problem i change speed sencor and it work fine now.
Thanks for the help