[CrazyAl]

Pinion angle changes from lowering or control arm adjustment are very small--usually just a degree or two tops. That small of an angle change is going to alter driveshaft length by a tiny fraction of an inch. There is no way that would ever cause issues with spline engagement.

By the way, the point is moot becasue the stock driveshaft doesn't have any slip-joint s in it or a slip yoke on the transmission. It has CVs. If instead you use a solid driveshaft (1-pc design) these are based on the Ford Ranger setup, and they have a very long slip joint intended for the longer suspension travel of the truck. Even if you were moving the driveshaft a lot (which you aren't) there would still be plenty of room.

Anyway, you certainly could use Fixed LCAs and an adjustable upper. That works fine, and there are people who do that.

I, however, recommend against it. I suggest adjustable lowers and leaving the upper stock (or if you are looking for upgrades, a SOLID upper). Why?

First off, the lowers are EASIER to adjust than the upper. Yes, there are two of them so you have to make your adjustments identical between them. However there is a lot more room to get a wrench on the lowers than on the upper. Adjusting the upper and then torquing down the jam nuts when you are done is a pain to say the least.

Second, an adjustable lower/solid upper setup is stronger than a solid lower/adjustable upper.