When my Dad had his Auto Tranny rebuild on his 1990 F-150 way back when, they said that the 1st sign to the tranny going out is that the speedometer falls to zero even though your cruising down the highway. They said to bring it in before you have to pay someone to do it for you, just less hassle if you can drive it in.

He had it rebuilt the 1st time when it went out, then the speedometer fell to zero after about 500 miles after the rebuild. He then took it back in and made them do it again because it was under warranty. I know thats not good news but, may be useful.