Agreed, either will do you a very good job.
As Rick points out the box is very simple and all parts are available to completely rebuild both the gearbox and transfer box. If you are loosing that much oil it's a fair bet that you will need to at least change the seals in the transfer box as well. Jumping out of second is as you note a common problem when the box gets worn it can be caused by any one or a combination of issues with the syncro hub, lay shaft thrust washers, input and output main bearings or excessive ware in the gear sets themselves, or of course the fact that it was never rebuilt properly the last time it was done.
The series of articles on the gearbox are well written on the G503, it's pegged to the top of the GPW/MB technical forum so you will be able to find it easily. If you do decide to do it your self use the truck manual and measure all shafts and thrust washers against the factory specs and replace if out of tolerance. It really is a waste of time and money putting worn kit back into a engine or drive train when you've got it stripped as the cost of new parts are both reasonable and available for MB's and GPW's.
Also don't forget there is a large pool of knowledge and experience on the Forum here at HMV so use the search engine and never be afraid to ask.
Good luck
Regards
Pete