Try pushing down before going over and up. Most likely and adjustment issue, worn linkage, or you have a 4 speed tranny.
http://www.4crawler.com/Diesel/ForSale/ShiftLinkage.shtml
See the white piece on the trans on the left that looks like a small milk bone almost. It is what shifts 5th, that one is actually broken and I replaced it with the missing linkz kit I linked above. 5th is the one that pulls on that and if it is cracked then it may not want to get into 5th.
Make sure you have plenty of oil in the trans. It is splash oiled I believe and if it gets low it will ruin 5th in no time. If it pops out of gear a couple times that is a sign of getting too hot and the next step is trans explosion.
If it just won't go in I would bet on the milk bone part though, I just didn't want to have you not informed of a disaster that can be prevented.
If you have it out as shown in the pictures, there's not much you can hurt by taking the end off the trans to make sure nothing has been cooked by low oil. In my experience a large amount of 020 transmissions are changed due to this. At this time you can test fifth to make sure it's working and even adjust it if necessary. If the teeth on usually the driving gear are gone, I'd suggest another tranny is needed or have to pull it apart and get all the teeth before using it with a new 5th gear!
thanks theman53 it sounds like a linkage problen like you described. it will go into 5th once in a while if you get it just right.
A buddy of mine just gave me a linkage rebuild for a gasser mk1 does anybody know if this will work in my 1.6d?
If it's a mk1 then yes, it will.