Peter
a double cardon joint with a normal uni joint one end of the front tailshaft WILL NOT FIX the tailshaft rumble on a 101 and in fact could make it much worse.
The only situation where that type of tailshaft set up will work on a rolled front diff and the double cardon fitted to the transfer case side of things.
The way you are currently running the after market tailshaft is most likely producing more vibs than the standard 101 tailshaft with a normal uni joint each end
the standard design goes some where near to getting a constant velocity effect from one end of the tailshaft to the other by the speed variations cancelling themselves out by having the uni joints at opposite angles to each other.
The other solution is a after market tailshaft with a double cardon joint each end giving a constant velocity effect no matter what the angles are.
a front tailshaft with double cardon joints each end will have a even more steeper angle of operation due to the extra lenght of the joints and will be weaker and very unsteady when worn.
Two CV joints like used on a current JK jeep tail shaft may work , but will have a short operating life on a 101 again due to running at steeper angles than they where designed for.
Note when CV and and double cardon joints let go they tend to not give much warning in a situation like that of a 101.

