2004 Chevy Blazer replaced transmission is a 99 Blazer it only shift first and second gear will not shift to 3rd or drive what is the issue thank you

 

If you have 1,2 and R, and when it tries
to go into 3 (or 4) the engine revs, then a likely culprit is a fried 3/4 clutc4-pack.

Do a google sesrch for 4L60E failures to check your symptoms against the different causes.