Hello, Landy newby here with '72 109 with a 2.5 turbo diesel in it.
I bounced it along a mile of rough track in (normal) second gear using 4wd for a quarter mile.
When I got to the end of the rough I let the 4wd off and continued but found the power well down on what it was before play.
It remains weak, doesn't especially like 4th gear and makes a noise / shudders a bit setting off in first.
Can anyone suggest what I've broken please?

 

Vibration and or rumbling : Check your prop shafts for play first, chock the wheels and release handbrake first or it locks up the rear prop then climb underneath and give 'em a twist. Check the UJ's on the props using a big, flat bladed screwdriver - poke the tip of the driver into the gap between the yokes of the UJ's and try (gently) to lever the yokes apart - if you get any movement at all, renew the roller bearings in the UJ's. These are the cheapest bits to replace.

If it's not those, you're possibly looking at something in either the transfer box or main gearbox - best way to tell there is, particularly if it's making a lot of noise, to drop the fluid out of each of them and check the drain plug for bits of gearbox (they're magnetic so you'll see it pretty easily).