-- Last Updated: Oct-10-12 1:39 PM EST --
I think someone mentioned dolly wheels to help carry the boat between the vehicle and the water. Also, arranging your rack to that you can put just one end of the boat up, then slide the other end up, cuts the lifting to a small fraction of what it is right now.
How much does your boat weigh? As others have said, there are plenty of very light boats out there, though they are not as cheap as the plastic ones. Let's say your current boat weighs 50 pounds, since plenty of medium-sized plastic kayaks suitable for smaller people weigh no more than that. Put dolly wheels on one end and you only have to lift about 20 pounds (it will be less than half the full weight on account of where the wheels are mounted) while rolling it to and from the car. To get one end of the boat onto the rack you'll have to lift a little more than half the weight (since it's not practical to grip it by the tip while doing so), but you do NOT have to lift overhead in most cases, or if you do it usually won't be by much. Then, lifting the end that's still on the ground is easier still. You'll be lifting less than half the total weight, and as you slide the boat onto the rack, the weight you lift becomes progressively less so that by the time your arms need to reach above your head you are hardly pushing up against any weight at all.
If your truck has a cap, you can mount one crossbar at the very rear, making this loading method easy. Otherwise, side-loading bars, which either extend out from one crossbar or run lengthwise between the two area easy for using the slide-up method too. I've always loaded heavy canoes and small motorboats onto the roofs of trucks and even full-size vans this way, and these boats are far too heavy for me to load in the same manner that I see used by nearly everyone loading kayaks. I use the same method with boats of moderate weight because I see no reason to do it the hard way. There's no reason the same can't be done with a kayak. You might think about equipping yourself with a useful rack before switching to an inflatable, especially since inflatables require a lot of time to inflate and deflate, and also to dry and store properly after use. You'll probably spend less money modifying your rack or getting a better one than you would spend on a good inflatable.
You can also invest in one of those racks that lets you load the boat at waist level, then swing the whole mechanical contraption up onto the roof with spring-loaded assist. Even THAT would most likely be cheaper than a new boat.
Oh, after reading your post again it looks like you are putting the boat IN the truck. In that case, putting some carpet on the tailgate will allow you to slide it in, in which case you only need to be able to lift half the boat's weight, mostly likely not even as high as your waist. Don't lift the whole boat - just lift one end at a time, and employ attachable wheels if necessary.