Lowering a truck makes the driveshaft think it's longer.
The yoke is splined so it slides back & forth as the suspension cycles. That cycling alters the distance between the pinion & trans tail shaft.
The splines absorb those changes.
A 4/6 drop doesn't usually create any driveshaft problems. Not saying that that isn't possible with that drop. Stranger things have been known to happen.
It's when going lower that the probs occur. 5/7+, bags, etc
The yoke can bottom out inside the trans tail shaft.
There is an easy way to check. Do this with the box back on, and after the cross-member has been notched.
The cure is NOT to shorten the driveshaft. Nope.
Shorten the yoke.
IIRC, my '98 has had that done, (4/7) but jeez, that was 17-18 years ago? I'd have to go pull it to take a look. Damn memory.
I do know that the D/S is still the stock length. Only thing I did was to have it balanced.
Once the truck is lowered & before removing the old u-joints, (You are installing new ones, aren't you?) take the complete driveshaft & wrap a zip tie around the yoke. Push it all the way into the tail shaft as far as it will go.
Pull it, wrap a second zip tie on the front side of the first one. Install the D/S.
Throw some dead eight in the box, get a couple of people in there & get the suspension bouncing. TRY to get it to bottom-out.
Pull the D/S. The distance between the leading edges of the zip ties will show the distance between the actual travel & the point of bottoming out. I'd be comfortable with anything over 1". You might feel better with a little more.
If it's less, when you go to replace the U-joints, a machine shop can properly remove the correct amount from the yoke (Removed from the D/S, of course.) on a lathe before reassembly.
High school/Trade school shops are good places to get small jobs like this done. For cheap!
There will be plenty of length left on the yoke to maintain a strong connection to the transmission.
Replace the transmission's rear seal before final assembly.
