transferbox knocking

hi guys after biting the bullet and doing away with my eas and coverting im well happy but have problem with transfer box its knocking under power untill 30mph then goes off,tried selecting low range gears orange cog light flashes then after few seconds engages into low range when i took it into specialist place he said mourne chain be comes stretched i have gearbox fault reading on dash guy said he has new transfer box there 250 quid is this bargian or is transferbox electrcal or mechanical problem