2000 Expy Dies When Shifted to Reverse

I have a problem with 5.4 dying when reverse selected with foot on brake. RPM will rise from about 700 to 800 and then drop to about 400 and often just die. Last work done tune up. Replaced plugs, pvc, fuel filter, tranny fluid and filter. Problem started after work done. Selecting drive with foot on brake get rpm rise to about 800 and then drop back to 700, no problem. Have checked voltage on IAC, EGR and DPFE connectors. Removed IAC and pintle shaft moves easily no grounding and good ohms through solenoid. Air filter clean. Again only happens in reverse. Any advice will be appreciated.:banghead: