Your reverser needs to be taken out and fixed. Or at least looked at and then maybe fixed.
I have lost count of how many of these I have worked on over the years. But enough to know what you are facing. From what I have read here and knowing what I know, your reverser has a bearing that has gone bad on the forward end of the Intermediate shaft in the reverser. When they go bad they will literally weld themselves to the shaft itself and to the inside of the forward clutch pack quill. This Quill also is the input shaft that is connected to the engine.
When this bearing galls or welds itself to the shaft and quill it then becomes one solid piece that goes all the way to the transmission. Sense these machines that have reversers do not have a driven clutch, your clutching action comes via an accumulator that acts as a clutch. This is all done hydraulically within the reverser valve body on the side of the reverser. This is why you couldn't shift your transmission into gear with the engine running.
Now, that being said! You say that now the machine appears to have corrected itself and you can shift it into gear and all seems okay. What has actually happened is, these bearings will turn loose from the end of the shaft or they will spin the bearing race inside of the quill and for a little while everything will function. But this is only temporary! And you will have issues big time! You should not run this machine any further and take it apart now and maybe you will be lucky enough to salvage some very expensive parts. Namely the forward clutch pack quill and the intermediate shaft. Hopefully for your sake the bearing has turned loose between the bearing and its race. Then maybe you can get by with only having to replace the bearing and race.
If you continue to run your machine the way it is, you are only going to hurt it further. It hasn't magically cured itself! Unfortunately they don't heal on their own! I don't want to come off as the bad guy here... I am just trying to save you some real expense. Which will be bad enough on its own but maybe you will get lucky and only be out some labor.
Now with all that being said... What happened to the oil for the reverser in the first place? It obviously leaked out or you wouldn't be here looking for answers. There is a reason somewhere that has caused the reverser to loose fluid in the first place. There are any number of ways for it to leave but they should all show up as a leak some where. This issue will need to be found out as well. Or you will just end up right back where you are now.
These 350 reversers only have about two and a half gallons of fluid in them to begin with. When you loose even a little of that amount you are at risk of ruining the component. The key here is maintenance.
I don't like being the bearer of bad news. I hope for your sake that this ends up good for you. In all my years of experience though, I have never seen a piece of machinery fix itself. I just don't want to see you make it worse than it already is. Good Luck!!
