I have a 2000 chevy blazer with the 4L60E trans and my reverse went out the other day after hearing a pretty loud clunk. The engine gets up to 4000 rpm when shifting from 1-2 and 2-3. When I put it into reverse I can hear a whining sound. I tried putting a high viscosity sealing additive in it and it didn't help. Any advice or input would be greatly appreciated.

 

From the "no reverse" to the "loud" cluck and high RPM on the 1->2 shift, I suspect your Sun Shell broke, which is a common problem with with pre '03 4L60E.
I suspect your don't actually have 2nd gear and the high RPM is caused by the delay to reach the MPH for the shift into 3rd.
With a broken Sun Shell you lose Reverse, 2nd and 4th.

 

I think your absolutely right. I drove it on freeway and it does not shift into od. It makes total sense to me know. Thank you for your answer. Two more questions, is repairing the sun shell a hard fix and is it ok to drive short distances?

 

You may grind metal up in the trans driving it like this the fix is not hard but does require a complete tear down, I would consider a full rebuild or replacement ,

 

Yeah, depending upon how and where the Sun Shell broke, it may be distributing ground up metal throughout the trans. This will require a complete rebuild with careful cleaning of every part. Some builders replace the planetaries if there is any sign of ground up metal.
Unfortunately the converter then also needs to be replace or at least opened, cleaned and welded back together. And any aftermarket cooler is best replaced too after throughly cleaning the cooling lines. Sorry for all this bad news.

However, since the damage is already done and you have since driven it a bit, driving it some more (easily!) probably won't increase the damage level.