My driveshaft is stock but I had it rebalanced with some new u joints when I did the 3:73. I read that the stock unit is ok to run with the T56(?)
if you don't have any vibes now with your 3:73/4L60E and do the T56 swap it will be the same.....IF you maintain the same DL angle. If you go 4:10...you may find (read likely) that you now have vibrations and need a larger diameter (rotational mass thing) and better balanced DS (DENNYS) and if so, IMHO, get 1350 yokes for it. This means you will need to change your pinion yoke but the Dennys DS will come with that if you order that option
Tip: before you unbolt anything and once the car is raised up to do the swap just measure your motor angle with one of those magnetic angle finders. Put it on the intake manifold and document whatever angle that is. You want the same once T56 is in. This may involve adding/deleting the shims that are on the car now (between tranny mount and tranny). You can also measure from the front u joint down to ground also to have a 2nd point of reference. Of course the car will need to be on stands and not moved so the variables of measurement are constant between the 4L60 and T56 position.
On the swap itself it did for me also seem intimidating. I have wrenched on cars since I was a teenager with my 1st car (57 chevy and I converted it from Powerglyde to Muncie 4 speed)) but do not claim to be a mechanic. Back in 2005 there really was only the BBHP "kit" or fabricate something on my own. I read the 32 page instructions BBHP had SEVERAL times and did the swap myself in my garage with car on jack stands (note get large jack stands to raise car high enough to get the 4L60 out and absolutely buy the harbor freight tranny jack). It is a very satisfying accomplishment and one job in my opinion you want to do yourself so you know every detail and can make the decisions of "do I cut here, do I mount this thing this way, etc. your self. Fortunately enough of us who have done it are here to answer ?'s and without a doubt Mikes effort in authoring the install manual is priceless and a very clear map on how to do this. In the end you will have a ride that is way more fun, IMHO, to drive and will function/look as if factory installed if done correctly.
