Outer Limits 4x4 Board

my 5th gear is blown, which sounds like a fairly common occurence. I don't know specifically what is wrong with it, it just makes a horrible noise when put in 5th, so I leave it alone. I have a spare box, that supposedly fits but hasn't been checked if it is the correct model yet (yeah yeah, working on RWC!), that is condition unknown also.

Do I
1) pull out the old box and straight up fix that? Its going to be a rushed driveway job on wheel ramps, is that a good idea with gearboxes? Never played with one, how hard can it be?

2) get the second one sorted whilst being able to still drive around and then do a swap?

3) take the whole lot to someone (suggestions in eastern melbs?) and get it sorted? How much $$ roughly is getting fifth replaced/box sorted?

The gear was broken by the previous owner and I don't really have a lot of details... But I know that not having fifth sucks

get second rebuilt could only cost a few hudred if its good they prob be able totell u there and then . a full rebuild inc new gears could cost up to 1500 dollars . but i would spend the dollars and go exchange i think marks adaptors do exchange and it prob wpuld be in the 1000 to 2000 range but i could cost that to rebuild.

does replacing 5th (if thats all it is?!) consititute a full rebuild, or would that be, as the name suggests, fully rebuilding all the gears, slectors etc.?

And how about the ghetto 'box swap in the drivway. Alll I have is some ramps and a trolley jack. What are my chances?? Is it easy enough to have a crack at with a mate, or is it gods own job? I'm guessing bolts at the bell housing, then not sure how it mates to the transfer case...? I'm sure I'm neither the first or the last to go through this- what have others done??

cheers.