1994 Chevy Suburban Any ides?

My suburban has been revving while driving in O/D for the past 4 months. If you laid off the gas you would be fine but the problem would persist unless you gunned it hard.
Now it is not getting into 3rd gear and above, at all. It will shift well through first and second but once it leaves second it never finds 3rd or 4th. It's like you have the truck in neutral and are stepping on the gas, and if not careful, red lining it.
Help please!