The lightning rear will be limited slip(not a locker at all) GM first called it a posi, to everybody else it's a ltd slip, which is for most common porposes your best bet. They are a large step above an open rear because, they have a series of clutches in the carrier that tighten up against the spider gears, pushing them together harder as the pedal is pushed farther( but the gears are not "locked" together in any way, all are free turning). on level ground, this helps apply pressure to both axels instead of just one, but they loosen off as you let off the gas to allow slippage when turning corners. but like an open rear if one tire comes off the ground or has much less traction than the other( 1 on ice 1 on pavement etc.) then the power transfer is much less and you end up with still 1 tire spinning in that situation, alot of happy pedal will eventually bring it to about a 70-30 split. near equal traction for each tire will result in both spinning equally, Your truck probably has 3.55 gears now so going to 4.10's will make quite an improvement in low end grunt but you will lose top end speed and probably a few mpg on the highway. But, if you have the overdrive tranny this may actually help to keep it in it's optimal power range thus making an improvement, depending on how fast you go and your local terrain.( your engine will be in it's power range rather than lugging below it) You will want to count the axel splines because on the F150's they can vary from 28 to 31 and if yours aren't the same, you'll have to change them too. I would assume your truck is 2wd so really unless you race it, a locker, spool or other devices are not going to serve you as well as a limited slip. it is the best general duty all around use traction aid you can put in. You could just hunt around for a complete rear end from any 87-up xlt or better f150 or bronco with a towing package as they usually had a ltd slip and most of the 4x4's had it in the rear. one last thing to do is replace the clutches on the rear end you choose, without good clutches it's still pretty much open. you can identify the rear ends by the tag attatched to a bolt on the diff cover (if it still has one) a ltd slip is identified by an "L" infront of the gear ratio ie(L3.55) as opposed to just a 3.55 marking on a standard open rear. It may be that your clutches just need to be replaced if they're worn out your rear end will act like an open, not expensive to replace. if the rear in question doesn't have a tag pop the cover off and look inside, you will be able to see the edge of the clutch discs on each side where the axels enter the carrier( should be 3 or 4 per side) and the numbers for the gear ratio can be found stamped in the edge of the ring gear ( biggest gear in the rear end look for 2 numbers). You take the larger number and divide it into the smaller one to get the ratio, ie you find 11 and 39, divide 39 by 11 and you get 3.545454. rounds up to a 3.55. or 45 and 11 will come out to 4.090909, rounds up to a 4.10.
If you do want solid power to both wheels all the time, no matter what until you let off, then put in an an auto locker( eaton and detroit make the best) they lock up solid when you mash the pedal(gears are locked together 50/50 split for power whenever the gas pedal is depressed no matter what) but also partially unlock when you let off allowing for some slippage while turningwhich is alot friendlier to the rear end and tires.( they do make a chattering noise while turning but you get used to it) Fords' 8.8 rear doesn't really like being locked up all the time. if you put in a spool, full locker or even worse linclon lock (weld the spiders) you will be replacing axels in no time at all. especially if you drive around town alot. Alot of vehicles drive around like that forever with no problems and yes there are a few exceptions but for one a fully locked rear makes for absolutely evil driveability, its very hard on drivelines and tires especially on dry pavement and with an 8.8 ford rear( whats in every f150 since 1980, factory) you will break axels, as to why I don't know, but I do know that it happens almost every time. I know it's alot of readin but I hope it helped. best O' luck