It sounds like you are wanting to do this work yourself. If you are okay paying someone else, and if you live anywhere near Indiana, you should visit a privately-owned, Rover repair shop called, PT Schram Rover Repair. Has all the diagnostics and tools, but without all the costs of being affiliated with Land Rover. Great guy too.
Without seeing your truck, I would bet that the vibration is the driveshaft. Any information you need to do the work can be found in a service manual, or possibly even here on this forum (I have not searched it). Try greasing the u-joints first, but you will probably have to replace them.
Will the lever for 4-low just not move into position (causing the truck not to shift into 4 low), or will the lever move into position, but the truck will not shift into 4 low?
The 3 amigos could be different things. You may need to go to a dealer, but I would recommend LandRoversOnly.com, forum. Look for PTScram, and he is very helpful (along with many others there). Tell PT that I sent you.
Hope this helps.
-Gary
"I can't believe I'm sitting here, completely surrounded by no beer!" -Onslow