I purchased a 2004 F250 6.0 a couple days ago and changed out oil, transmission fluid and coolant flush and fill up. After doing so I found that the truck does not go into reverse and that neutral is acting like drive gear. Note that prior to fluid swap it was running perfect in all gears and had no problem with reverese. Thanks in advance!

 

Perhaps the gear shift linkage is out of adjustment.Did you actually service the transmission or someone else?

 

This is one way to check this.

Disconnect the shift cable from the trans down by the trans.
Work the lever on the trans through the ranges,,, park, reverse, neutral drive and low.
You will notice a longer distance of travel when you push that lever up into the park position.
Now go inside the vehicle and make sure the shift lever is locked in the park position.
Adjust what ever you need at the cable connection that connects to the trans so the connection fits exactly with the lever on the trans
Now you know the trans and lever are both in park. Make sure the connection is tight so it can't go out of adjustment again.