I had my truck at a spring shop to install degree shims in the front to add some caster.

I wanted them to install 4° shims.

Long story short, the installer called me and said 4° was too much - that it would cause
too much driveshaft misalignment… so now I want to know how much misalignment angle is acceptable.

Now the long story…

The truck is about 3 to 4 inches higher than it was before - I had X & C code springs installed
(custom made springs rated same as F350 X-Codes front, C-Codes rear).
So I figured adding degree shims would adjust the pinion end driveshaft angle to almost straight.
The thing I didn't know is that it is better to have the driveshaft at an angle and have the pinion
shaft and the Transfer case output shafts parallel to each other rather than have one end of the
driveshaft at an angle and the other end straight in line with the axis of the shaft.

When they were doing the degree shim install, I got a call from the spring & suspension shop.
The mechanic told me that if I put in 4° shims my ujoints would probably break next time I use 4WD.
I let the installer talk me into 3° shims instead of the 4° I originally asked for. The 3° shims increased
the caster enough to help quite a bit with my steering, but I wish I'd gone 4° because I would like to
have gotten that extra 1° of caster (so it’d be even more better). I don't think it would really be all that
bad to have the pinion shaft and the Xfer case shaft out of alignment by only 4°, and I was hoping I
wouldn't need to change the camber/caster bushings to get more caster. Right now, it’s much more
driveable, but just a hair farther would probably have been perfect.

Can someone shed more light on how critical the alignment of the Xfer Case Output Shaft and Differential
Pinion Shafts really is? Do they need to be parallel to each other? How much misalignment is acceptable?

BTW, for background info, so far I have replaced
Springs (Front X-Code 3500 lb & Rear C code 3000 lb).
Urethane Spring Bushings
Urethane Track Bar (Panhard Rod) Bushings.
Front Swaybar end links (and urethane bushings)
Front Swaybar mount urethane bushings.
Ball Joints
Steering Stabilizer (Monroe Gas Magnum)
Bump Stops (Urethane, Front & Rear)
Added a heavy duty Helwig Rear Swaybar.
Replaced all 4 shocks (Monroe Gas Magnums)

The steering gear (box) has been replaced twice.

Most of my steering wander is gone, but it still has room for improvement.
I may go to a Redhead Steering Gear and new PS Pump.
Need to check steering input shaft from steering wheel to PS Gear (rag joint & U-Joints, etc.), and I still might
replace all of the tie rod ends… I found a complete steering rebuild kit with all Moog parts which would eliminate
almost all possible sources for steering freeplay. But, I also found that it’s cheaper to buy all of the parts in the kit
individually on Amazon.
I probably should check the condition of the splines on pitman arm (don't know if there's play - Redhead would probably
check that for me if I buy one of their steering gear boxes)
And then maybe re-do Ball Joints with Moog ball joints, especially if I do find a less expensive source for the complete
steering rebuild kit and the ball joints are included.

I'm determined to get it perfect. I can keep the truck in the lane now… it’s improved a lot, but there’s still some
vagueness in the steering. I'm not sure if it's freeplay, or maybe binding a bit, but it still doesn't feel right.

 

From what I gather reading this, you front springs are leaf springs. The shims will definitely change caster angle and pinion angle. You should be using shims to correct the pinion angle, my preference is to cut and reweld the mounts vs shims. The caster is corrected with the bushings, if you're past what a bushing can offer the inner C needs cut, turned and rewelded, but you would need that step. If you have a single cardian shaft, one joint at each end, the pinion and t-case should have the same angle to within 1 degree. If you have a double cardian, a single joint and the other end 2 joints, the single end should be in line with the pinion within 1 degree and 2 joints at the t-case take all the angle as long as it doesn't bind. This isn't something you can eyeball, you need a good preferably digital angle finder. Most joints are only rated for about 5 degrees but will still operate beyond that angle. Once you get a few degrees mismatch you will get vibration.