Is my tranny going out??
 

I have a 1999 Chevy blazer with 158xxx. When I'm driving on the highway when I get about 50-60 the truck feels like it is rocking back and fourth. Sometimes I feel it at about 40 as well. When it does this if u watch the rpm dial it bounces up and down. After that it seems to do ok. Is my tranny going out it do I just need to give it a tune up?? Any help is gratefully appreciated.

You'll need to have the electronics diagnosed on it. Drop it off at the transmission shop and they'll tell you whether the TCC solenoid is good. If it is, you either need a new torque converter or a transmission rebuild.

Would a sign of it being bad be when u put it into reverse and it rev up

that would be a sign of a bad tranny.... or at the very least, valve body.