Imagine a line from top to bottom directly through the center of your front hub. This is the axis your steering turns on. You typically do NOT want this line to be 90 degrees from the level ground. Depending on the vehicle, tilted slightly either forward or backwards. This is your caster setting
When you lift the 80, even slightly, you don't (typically) change the point at which the front axle is attached to the frame (the rear bushing on the radius arms), so this causes the front end to arc down & to the rear; thus changing the caster angle (and in turn, the pinion angle).
On a typical standard drive shaft, you want the angles on the u-joints to be the same at the axle AND the transfer case. As outlined above, when you lift the suspension, this changes the angles; but not uniformly. Sometimes this can manifest as slight vibrations in the driveline.
To install the caster plates (probably the cheapest & easiest attempted fix) you just unbolt the radius arms from the front axle (one at a time) and install the adjusted plates. These plates attach directly at the rear radius arm mount on the axle; then have two stacked holes at the front end. The upper hole at the front gets attached to the axle, & the lower hole gets attached to the radius arm. This effectively raises the point at which the radius arm attaches to the axle; and in turn, rotates the pinion down a couple of degrees.
There are guys that'll tell you "take it in" & "get it professionally aligned"; I'm NOT one of 'em. It's not like our rigs need to be "dialed" in, in respect to the front end alignment. There are only a few options in regards to caster adjustments & it's either adjusted or it's not. You can pick up the caster adjustment plates for about what a front end alignment is gonna cost ya, & if the "professionals" say your caster is out of spec, they're gonna tell you to get some form of caster adjustment installed.
So, why not just cut out the middle man (& the extra expense) & use the trial & error method? Spend a Saturday & slap the caster plates in, then test drive it. If the vibes go away, great; you won't need the $400 DC driveline.
If they don't go away, test drive it without the front driveline. If it handles BETTER & seems to track straighter down the road, leave the plates in & get the DC shaft to correct the vibes.
If it handles WORSE, yank the caster plates & sell 'em on MUD for 80% of what you paid; then get the DC shaft. Everything will be back to the way it was before you started this endeavor & you're just out a little cash, but you've increased you knowledge level of your Cruiser.
Easy Peasy...
