1999 Chevy Blazer

Is the abs light on, how does it shift from a cold start?

My 1999 chevy blazar will rev up to 3000 rpm, shifts hard then takes off and my speedometer goes back and forth. This happens cold or warmed up. Yes the ABS lights are on.

Hey I have a 98 blazer mine acted the same way I had it scanned and found it to be the speed sensor on the left side of the transfer case that sensor if it goes bad will make your tranny go crazy it did mine
i would change that sensor my ABS light was also on make sure u disconnect the battery before u change it.

Let me know how it goes

Thank you for your help. We d/c the battery, changed the sensor. Now the transmission only races intermittently but not as hard. However the ABS light is still on? Any thoughts on that?