I've heard of 4x4 guyys who go through driveshafts like every two weeks because they jacked their truck up so much and the pinion angles are so terrible. I have no idea about suspension travel and driveshaft length, though... but I do know that on my '78 Dodge, the transmission end of the driveshaft is designed to just slide back and forth in the tailshaft, and on my '55 Chrysler it does the same thing, but with a ball and trunnion joint.
well, as long as the pinion angle and the angle of the tranny output are the same (and the angle is not too extreme) there should be no problem. After I lifted my Jeep (Ummmmm, Gasser style ) due to the short driveshaft I had to get a fixed yoke adapter and run a CV shaft with the pinion pointed way up. Here is some in depth driveshaft stuff http://www.4xshaft.com/driveline101.html
when i lowered my truck i had problems with the driveshaft rubbin a cross member so we just c notched the cross member so far no problems just a small rub when i turn to sharp real fast but hopefully new shocks takes care of that problem
haha, who'd a thunk it its a 4.5" suspension lift. longer trucks can go that high or higher with no real driveshaft woes, its just that the jeep is so damn short. before I did the fixed yoke kit and the CV shaft the u-joints were bound up so badly we couldn't even move the jeep, it was nasty. With my limited driveshaft knowledge I'd say you could go 6 inches no problem as long as you kept thing lined up correectly.
the LAST thing i'd be worried about is DRIVELINE ANGLE...fuck,,,,id be MORE worried about asking QUESTIONS about welding that staight axle "straight" under my car than anything else...