I just got a 2001 blazer 4x4 and it has transmission problems. I can go into reverse and first and second but it won't do nothing for 3 and drive I'm wanting to know if it could be a simple problem before I put a new tranny in it

 

It is likely you have a blown 3/4 clutch pack. This is not an uncommon problem with old transmissions and/or ones that have been overheated. This is what happened to mine. I had to have it rebuilt.

 

Is the trans fluid dirty? If so, the filter may be plugged, an old filthy system must be FLUSHED, including torque converter, lines and radiator trans cooler.

I think its likely if it is dirty its probably too late.