Won't engage reverse

I have a 1994 Ford Explorer with 155000 miles. After driving for about 30 minutes at 55mph I stopped at a store. The engine was off for about 10 minutes.
I shifted into reverse and the transmission would not engage into gear. It would shift into forward. So I put it into neutral and my son and I tried to push it backwards. It would not budge an inch. I checked the tranny fluid, it was at the proper level. I rubbed some between my fingers to check for grit, there was none. The fluid did not looked, smelled or tasted burnt.
I am an outboard machanic and I have seen my fair share of wierd things happen butthis was a first.
Lucky enough a man with a truck and a tow rope pulled in and dragged me back far enough to allow me to drive away. I drove it around the block about five times listening for any odd sounds. I did not hear any. Being over a hundred miles from home I did not have much of a choice but to head for home. I did not have any problems the whole way home. When I got home I tried putting it into reverse and it still will not engage.
So, now please tell me what can be wrong with it? I am thinking that it could use a good flushing out and a new filter screen. Any help you can give would be greatly appreciated.