occasional stickin of r380 gear shift

[cooter]

it seems to happen if i get wheels skiping under load on rocks or reversing out of steep driveways in the wet (i have a part time conversion)
i have 1st 3rd and 5th no 2nd and top
i have thought it could be a worn selector
after a little while it can be reefed into second and it drives normal untill the wheels skip badly again (it also happens if i use compression braking in 2wd down slippery slopes) any ideas
the box was rebuilt20 000klms ago

[Pedro_The_Swift]

"part time" as in rear wheel drive only?

[cooter]

yep untill ilock the hubs and select 4wd (what used to be cdl)