Hello Sparg,
I too have the pleasure of owning a Defender 110. When I first got it I went off on a Road trip to sunny Cornwall. I noticed a vibration at high speeds but all was fine at average speeds ( 40 -65 mph). I thought I should check out for loose prop, warn UJ's and dutifully lay underneath shaking and turning bits and bobs but all seemed fine.

I spent a lovely time that week up until the last day I was there ... This was the day that the prop came adrift , as I was pulling away up a hill ( Engine under load) went straight through my drivers floor panel just missed my foot!

My how suprised the wife, kids and dogs were ! Bang ( Lots of dust). Limped home on one prop but all ended well , a new Prop and UJ's later

I reckon check your UJ's in the Prop ( most likely front one)

I would suggest looking at the sliding splines on the front prop , as i had similar probs on my 110 at autobahn speeds ! , and thats where I solved it , they tend to get a bit sloppy and then try and rotate oval , always greased it regularly . I also fitted the damper as on disco (bolts to front diff) NVH improvement ? JMHE

Mine tops out at 60mph lol

if you have doublejoint front prop, i had similar fault, i tried 3 new front props and all same... then one day i see aligment marks on prop, turned like this: arrows pointing directly: and BAM! that was my case, a like magic, it disappeared! others say that those dont need aligment but u never know. it was very bad at 110km/h and up to 122km/h.