Headlight lenses are clouded.

Chances are, it's pitting on the outside of the lense, not the inside. The inside probably is clean UNLESS, the rubber boot covering the holes is missing. You'd have to pull the headlight out and off the truck and try and clean through the holes the bulbs go into.

The headlight cover is not really a cover...it is the assembly which needs to be removed completely from the car to access the bulb hole that is the only access to the interior surfaces of the assembly. Lots of screws and awkward positions to get it out. Best bet, leave it alone or pull the bulb socket and use very long q-tips to clean out moisture!

They can be adjusted up/down. see picture for adjustment screw.
The headlight housing can leak and condensation will form inside and maybe stain the inside. It is very difficult to take the housing apart... do go there.
It's more likely that that outside of the headlight is discolored with age. There are kits available at parts stores to recondition the outside of the lense.
The bulbs will also get dimmer with age. I recommend Philips VisionPlus; 9006 for low beams. You can get a pair on Ebay for $20 or less. They'll be $40-50/pr at the parts store.
This headlight is the easiest I have ever seen to remove. The headlamp can be easily removed by pulling up on the two white strips about a half inch (??). Then wiggle and pull the assembly straight forward.

You cannot remove the lense. it is factory sealed. only access inside is by removing bulb from rear of the lamp assembly. Your problem is probably a degraded hazy lense and not the inside. have lenses cleaned by a detailer or shop that can do a lense restoration.
Good luck

There is no taking the lense apart. It is a sealed unit. If the lense needs cleaning on the inside it is because the headlamp assembly has been damaged and needs to be replaced. If it just has the bad yellowing on the outside, most auto parts stores have headlamp cleaning kits available.
AutoZone lists your headlamp assemblies at $299.99

It might be a good idea to check with a salvage yard first. You may find a good used one for less than half the price.

I've never seen the problem on the inside of the lens, only on the outside. Auto parts stores sell a kit to clean the lens.

pull off the connector from the back of the headlight, pull off the rubber seal, press the springs together (either side of where the connector plugged on) hinge the springs back (they will most likely spring open themselves) take out the bulb (have a piece of clean cloth ready to put the bulb on, since if you get it dirty it will shorten it's life from a few years to a few months!!!) , now what you need is some string - a length of stiff wire and a peice of soft clean cloth - tie the cloth onto the end wire with the string (over the edge of the wire so it doesn't scratch and securely so it wont fall off - it's hard to get it out!) and poke it into the hadlight and polish up the inside, you can use a few drops of washing up liquid and warm water if it's stubborn. use a new piece of cloth every now and again and you should end up with a nice shiny light... you might have to do the other side to make it match...
be patient because it is fiddly but it will work, at least this way you dont have to take the light out of the car and mess the aim of the light up.

First of all, the best if to NOT remove the covers if you are really talking about the plastic lenses on the headlights. They are glued on with a special adhesive, and usually you will break them into millions of pieces before you can get them off. Specially that its very very rare that the lenses need cleaning from the inside since the inside doesn't see much dust or sand or anything. If you need to clean them because they are now yellowish and/or hazy and don't let enough light out anymore, best bet if to just go buy some headlight lens restoring compoound, which can be bought at most car part stores, and i do believe Wal-Mart sells some also. With that stuff you need 2000 grain sand paper to do the rough of the job, and finish up with the paste/compound you bought to get them all nice and shiny again. If you do the job right it usually brings back at least 80% transparency and will have cost you around 20$ US in the worst case. If not, best bet would be to buy aftermarket OEM replacements like the TYC company sells. Usually though they go for about 100$ each, and if you replace both left and right your looking at more or less 200$, so its alot more expensive, but you will have brand new headlights that will last longer and give more light than using the paste/compound i was talking about.

You certainly can try taking out the hdlt. and make sure the bulbs are out. You can rinse the headlight out with water and then let dry. Usually when you put the light back in the heat will actually dry the rest of it out. You can also you some buffing compound and clean the outside of it.

its really easy. Open the hood, you will see two metal flat bars on the backside of each headlight, holding the headlight assembly on. Pull up on each of them.... remove headlight. Ta da.