blimey...simples...to find out where vibration..remove rear or front prop.put in diff lock drive it..no vibration,you removed offending item..too cure..
remove your poo rear prop and rear diff output flange..fit proper metal4 bolt type.second hand is fine!
be a cheapscape and go buy ex range rover off ebay etc..or blag one..i have 3 spare props,come and get one.£0
then..and the most likley cause for vibration..remove front prop..junk it..go buy td5 front prop off ebay..(£50+)has double cardon joint..go buy front output flange for lt230 to match (dont fit yours),ashcroft have them,not expensive..fit..job done..
THEN as you will discover your car is crap handling at speed,tramlines,dont self center on steering properly..so go buy qt or similar..NOT ****E..front corrected radius arms..also if not done cranked rear trailing arms..
can all be done cheapish,(except arms)