As above, a problem selecting gears is a clutch, main box issue.
Transfer box, put the diff lock in and see how much movement you have at the flanges [both props need to be off to do this properly] If there is a lot of slop when one flange is...
Selecting gears will be clutch related, the drivetrain will all wear together so g/box to t/box then t/box to diffs..halfshafts ect
sounds like a strip down, if there's an inch of play then its time to get your hands dirty.
Well all, I bought a defender 110 year 2000 td5 non runner, have since got it running and driving but is grinding and clunky to drive and struggles selecting gears, I have checked underneath and the transfer case has a lot of play on the output...