1966 Muncie M21. New synchros, bearings, gaskets and seals. Shifts great, Linkage adjustment feels spot on. 442 Shifter rebuilt, no vibration of stick or drive line. 80-90wt dino fluid full. Wants to pop out of 4th gear upon decelleration. Goes back in to gear fine, but still wants to pop out. When I get under the car and move the linkage, it sounds and feels like it goes in to 4th just like 3rd. Help.

 

Get a new 3-4 synchro hub and check crowns on the gear if they have too muck angle pick up a gear as well

 

^^^ This. Even though it might have been rebuilt and looks good with the side plate off, if the syncro hub and 4th gear nubs are worn, nothing can be done except replace it.

 

I ordered a new Tork Lock slider kit today. Put the car up on stands, gonna tear in to it tomorrow. .

 

Be sure the clearances between the brass synchros and the gear are correct when reassembling. The kit should come with different thickness lock rings.

 

When a transmission pops out of gear it is normally caused but a worn slider and bad small engagement teeth on the gear, in your case the input gear.
Here are some pictures that might help.
These are from a two articles I wrote, disassembly and inspection of a T-10, which is very similar to the Muncie.
Joe

 

Wow, great pics and explanation. My old one and new ones look just like that. New slider kit installed, issue resolved. Thanks to all who responded!!

 

Pilot bushing in crank. If input wobbles enough it will wobble hub out of fourth but not in other gears.