EDIT: Do the easy thing Joe mentions in the next post first. If that does not fix the issue then here are a few more suggestions.
In addition to the alignment and worn rod service letters linked below there are others relating to various lockup issues you might also check out. Go the service index at the left menu and then to the 37-42 year and then transmission category.
To narrow things down, if you disconnect the clevises and rods going to the transmission again and shift just the idler assy does everything work OK. If it still does not shift something has probably fallen out of position in the idler assy when it was disconnected and moved aside so you will need to investigate there.
If the idler does shift the next step would be to make sure the levers at the transmission are both working. Run each lever thru the range to verify all 3 detent positions on each lever are felt and nothing is worn in the detent assy then leave both levers in their neutral detented positions. Not likely the issue here but sudden detent and interlock problems have been known to happen.
If you need or want to do a linkage alignment, Clipper and later units had a hole thru the two shift levers at the base of the steering column where a rod or drill bit could be inserted to keep the levers locked together while adjustments were made. I think conventional bodies had the arrangement but am not completely sure so check and lock them together if possible so everything stays in the same position while you do the next step.
With the gearshift lever at its usual neutral position and some method or someone is keeping it horizontal lift a rod and clevis assy back up and see if the clevis pin will go into position. If the clevis did accidentally get moved it probably will not slide in and you will need to adjust the clevis until it does. Then move onto the other lever and do the same thing. When everything is connected again remove the alignment pin from the upper levers (if it was used) and see if everything works.
Here are a few service letters with alignment instructions and showing the shift lever and idler assy.
https://packardinfo.com/xoops/html/downloads/SC/SL-VOL14NO21.pdfhttps://packardinfo.com/xoops/html/downloads/SC/SL-VOL17NO5.pdfhttps://packardinfo.com/xoops/html/downloads/SC/SC-VOL19NO11.pdfThis Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=247702