Bob,
Your truck uses the 4R70W automatic transmission (as I recall). They are KNOWN for suffereing from "shudder", which feels like driving over "rumble strips" w/o the noise.
The "fix" is to have the ATF changed and replaced with "Mercon V" ATF. No, an addative to "regular" ATF is NOT acceptable.
There is a DIY "How to" article here:
www.crownvic.net/tech/transservice.htm
Oh, be SURE to disconnect the battery for 15 minutes or so ANY TIME you perform an ATF service! This will re-set the "adaptive strategies" in the computer for the trans.