i would definitely get the bigger shocks if you are going to use the tt and shackle / add-a-leaf lift, when you can afford it, one reason is because it is increasing your suspension heigth and by doing so you are increasing your articulation so to truly let your vehicle be able get the those wheels up and down the full capacity that the tweaked suspension will allow you will need a shock that can match that longer travel of the suspension. to be honest with you i went 10k before i got new shocks so its not going to kill you to not get them at first. the reason i didnt replace my bilstiens with bilstiens was due to availabilty, price, and my name is gabriel and i always wear a "gabriel" hat so i wanted the gabriel shock. pretty silly really, but they are excellent shocks. one thing that you can find at pep boys and advanced auto is shock extenders...BE LEARY. for the ront shocks it is just a stud extender that totally weekens the shock, but the rear my friend has the extenders on his toyota PU and he loves them, it is basically an aliminum block with holes for longer bolts, it doesnt stiffen the ride at all, but a capible offroad machine, in my opinion, should have maximum shock response and a stiffer shock seems to respond better when going over something liek a tree or a rock pile (we dont do too much rock climbing in FL but we do have gravel pits and lime mines that i get to play in once in a while) so maybe the extenders are worse but that is up to the individual. Your shocks should last 50k in normal driving conditions. it sound lik you are not killing your machine or rock climing or anything extreme so you will probably be alright. you have bilstiens now? thats what 95xlt came with and they are great shocks. Ford seems to be pretty good with their selection of shocks for their vehicles.
good luck,
gabe