on the front drive shaft theres alot of other factors that come in to play with that such as your drivetran lenght the later model trannys where longer than the early model trannys, how far forward your axle has been moved your amount of lift and amount of droop in the suspention are all going to play a role so theres not realy a yes or no answer to that one
about the 23spline if your going duals and 4.7's at the same time use the factory 21spline input for the crawl box and then get the 23spline mco7 or mco7r10 or what ever you choose and then get the 4.7gearset in the 23spline for the rear case, the 23spline gearset and adapter are the same price as the 21spline units if i remember correctly
on the other hand im running duals with the 21spline 4.7's on 39.5 iroks and i beat the unholy crap out of it and i havent had any issues with the transfer case part of the drive train(knock on wood)
the twin stick wont do any good in the front case the front case only has a high, low, and nutral its just he passenger side of the J shift pattern so you would want that in the rear if you deside to go with the twin stick