No reverse

Would it shift into reverse before you worked on it?
If so, I have heard of this problem before on different makes and models. Was the transmission fluid exchanged under pressure? If not, sometimes it may take awhile for the transmission fluid to reach all parts of the transmission, especially with older vehicles.
I am not a transmission person. There could be other issues.

Yes it was shifting into reverse before the oil change. I let the truck sit for a few hours to cool off before working on it. This style of truck has a transmission oil drain plug, which I removed and oil drained slowly.

make sure the filter was installed all the way, sometimes the o-ring if it has one does not seal right and the transmission sucks air.

Thanks. The filter is held on with three bolts. I tightened them up pretty snug.

pull the pan off and check gaskets where arrow points, if broken or cracked that is more than likely your problem no matter how tight you made the bolts. Or if the old ones were left on and new ones installed.

Thanks, will pull pan off and check gaskets. Thank you, again