My 99 blazer has lost third and fourth gear but is fine otherwise. I can drive it to no end in 2nd gear and it has no problems. I replaced the output speed sensor and it helped it for a brief moment literally about a mile before it began slipping again. I'm hoping I don't have to rebuild it so I'm wondering what anyone has to say that may help

 

Sounds like your gonna have to bite the bullet on this one... every thread I've seen start this way, ended in a rebuild...

 

Sounds like what happened to me when my 3/4 clutch pack wore out. This will be caused from overheating and a seal leaking. A transmission shop can do a better diagnosis.