1977 k10 suburban

[Rayn9055]

My suburban is a all time 4wd I went to start it and let it warm up and I shifted it Into neutral to let all fluid to cycle and my vehicle warm up after a bit I went to shift into into reverse and then I started to free roll forward and no gears will engage. Every time I try engage gear it whines and after about 30 seconds stops whining and whines in any gear I try to shift into and all gears stopped working at same time the transmission is a th350 fluid is at right level a little dark red color and and looking for ways to troubleshoot my problem I am am planning on changing fluid and filter and was wondering what all else I can try to see if is a easy fix or not that may get it driving again. Any help please????