The front drive shaft has been removed from 1989 Toyota Land Cruiser by previous owner & gearbox may have been changed. What is the length / part number / details of the original front driveshaft please?

Please check the owners manual, it is all in there

Usually worn parts in the gearbox, called 'detents'. These normally hold the gear engaged. Try holding it in gear with the lever. If that does not work then the wear may be further in, on the shaft spacers and gear teeth themselves. You may be able to get a reconditioned gearbox, depending on where you are.

Another cause mentioned by owners is worn engine mounts, so check those too.
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follow this link to register at toyota online and down load a owners manual which will give all the spec's, quantities etc ( there free )
http://www.toyota.com/owners/web/pages/resources/owners-manuals

May be low of fluid.

The front tail shaft of the cruiser is prone to spline wear and failing U-joints. have the drive shaft inspected and replace if it cannot be repaired

try ebay.....

DID YOU GET THE BANDS ADJUSTED ON THE REPACEMENT BOX?BECAUSE IT SOUNDS ABIT LIKE IT ISNT THAT MUCH BETTER THAN THE OLD ONE POSSIBLEY A BAND ADJUSTMENT MAY HELP

Me and my mate did mine in 12.5 hrs

The Landcruiser 1HD engine uses 9.3 liters of oil with the filter. The oil is replaced every 5000Kms.

remove the carpet on the floor, remove the gear shift lever as well as the 4wd shift lever, remove the drive shaft (both) remove the bell housing bolts, remove the starter, place a hydraulic lift jack below the gear box , then remove the wiring connectors , then remove the cross member , then pull the gearbox out from its positin, drop the gearbox very carefully, when gearbox is on the floor, make sure its sittin on a up right position so it dose not spoil any of the elctric connectors especially the reverse light switch...(that seems to be the easiest way to remove a manual land cruiser gearbox) hope that wil help....