I think each drive shaft is balance to it's self, I don't think they are all bal the same so mixing a front to a rear of a different drive shaft may not fix it, it may make it worse, buy a used D S , around 100, also to replace the u joint is also around 100 (if you can find a u joint.
I had 4 mechanics plus myself check mine, I checked it 3 times myself. all looked good,, then i said i'll find the problem, burn out 3 times got worse with each burn out, then on the lift, and still felt good but when i turned it i saw the u joint coming apart, replace the D S problem fixed. jim