check the vacume line to the trans if that is good change the vacume valve that the line goes to.
YOU CANT START A NORMAL AUTONATIC TRANSMISSION IN GEAR, HOWEVER YOUR DASH INDICATES THAT YOUR NOT IN GEAR. ONE CAUSE COULD BE DAMAGE TO THE LINKAGE BETWEEN YOUR TRANS AND GEAR SELECTOR. THIS WOULD INDICATE A DIFFERENT GEAR THEN THE GEAR SELECTED. YOU MAY ALSO CHECK THE PARK PINS IN YOUR TRANS.
If its a older trans sometimes the vacuum hose comes off the trans solienoid.
You would need a ring gear for an automatic.
5th gear synro is inside the trans... the clutch has nothing to do with it.
This problem could be the linkage in the transmission. The transmission could also be going out and would need replaced.
your trans axle has nothing to do with you losing 2nd gear
If it isn't slipping it is downshifting to 3rd gear
In the transmission tail housing where the speedo gear would be in an older version.
It is a switch that indicated what gear or gear range the trans is in.
If trans is jumping out of gear, that particular gear is worn from use. The beveled gears wear until they can not physically maintain contact. Split tractor at bell housing and again at rear differential to remove trans. remove top of trans and shifter. Remove main and counter shafts and replace worn gear and any bearings that show wear.
Low trans fluid, or internal trans failure.
Have the trans computer checked for trouble codes. That will give you a place to start diagnosing.