An alternative... get a 50D... same sensor as your current camera, but in a much nicer body. Pigeon Point Lighthouse Morro Bay Harbor
You tell me... which of the following was done with 15MP 50D and which was done with 21MP 5DII?
Both shots were made with the same lens and even the same filter. But one was done with 50D crop camera and the other with 5DII full frame. I know it's pretty hard to tell which was which at Internet resolutions, even with some tighter details of each. Heck, I mistakenly thought the 50D image had been shot with the FF camera when I was doing post production work on it, getting it ready to print. You'll have to take my word for it that it's nearly as difficult to tell them apart at 16x20" print size, though at larger than that it will gradually be more apparent.
My point, there's not as much difference between the better crop cameras and FF, as some might want you to believe. You have to print big... really big... before it shows up a great deal. Now, I'll use the 5DII about two stops higher ISO than 50D. And the 5DII will still focus, even if a bit slowly, about 1 EV lower light than any of the crop sensor cameras. Plus it gives me a bit more control over DOF with the lens aperture. There is a little more fine detail straight out of the FF camera, a little wider dynamic range. But good lenses on a crop camera can come darned close. And, on the other hand, for sports/action I'd much rather have the 50D... Using the cameras exactly the same way with the same lenses, the cropper tracks movement better, giving me far fewer missed focus shots.
The 50D has a weaker anti-alias filter that I think allows it to render closer to FF quality, right out of the camera. I can tell the later cameras' 18MP crop images more easily, those cameras all use a much stronger AA filter (stronger sharpening deals with it pretty well, though). If you need video, the 50D doesn't have it, but the 60D does. On the other hand, if you are a pixel peeper and want to be able to Micro Adjust your lenses, the 50D has it, the 60D doesn't. The 60D also doesn't have a PC sync socket for studio strobes, while the 50D does. Of course, if using radio triggers that doesn't matter at all. Both cameras sync speed with a lot of studio strobes is 1/160 (some studio strobes allow a little faster... and portable flash sync is 1/250, a little higher than your camera... they also have 1/8000 shutter speed, and a shutter rated to 100,000 clicks, compared to 75,000 in your T1i). And, the 60D has that cool articulated viewfinder. By comparison, 5DII offers 1/8000 shutter, but 1/200 portable flash sync, 1/125 with many studio strobes. It does have a PC sync socket, but doesn't have a built-in flash (if that matters at all). The 5DII has a 150,000 click rated shutter, better than either of the croppers and double the durability rating of your current camera. It also has Micro Adjust... And can shoot HD video. No articulated LCD monitor, though. All three have Live View, Mirror Lockup and most of the other features. All three use 9-point AF systems.... The 50D and 60D are more of a step up from your camera's, while the 5DII's is more similar.... I.e., single cross type at the center, though the FF camera is unique having six add'l and optional "assist" points right around the center AF point, though they only work in AI Servo mode. The 50D and 60D AF seems identical, and all 9 points are cross-type. 50D, 60D and 5DII all use the same processor, Digic 4. The 60D has a newer, 63 zone metering system (same as 7D), but it isn't hugely different from the 35 zone system on the earlier cameras.
Frankly, I also don't feel there's a whole lot to be gained going from 28-135 to 24-105L, other than 4X or 5X as much money tied up in the lens. IQ isn't all that different at the focal lengths they share. All the way out at 135mm, the 28-135 is just a little soft.... and it's a slower f5.6. And there's 2 more mm at the other end, which isn't insignificant. But the 24-105 tends to have stronger vignetting at the wide end, more noticeable on FF cameras than on croppers. The biggest difference between them is probably better IS on the L, as well as better sealing, better build and greater durability. But, buying them used, if needed I could replace 4 worn out 28-135s for what it costs for one 24-105, which I'm not sure would last 4X as long. I've used three different 28-135s over the years and haven't worn one out yet personally, though I have heard/read reports of them getting a bit loose and floppy after extensive use. The 28-135 does tend to have "zoom creep" more often, but it occasionally happens with the 24-105 too.
I don't know what other lenses you might be interested in. There are literally dozens of possibilities. All three of your current lenses can be used and will perform the same way on 50D or 60D, but would behave quite differently on a FF camera.
Don't get me wrong, 5DII is a good camera and I happily use one. I just think there's a lot of hype about "full frame goodness". It would likely serve your needs well, as you've described them. My intent is to try to be unbiased about the comparison, though. After all, the FF camera is going to cost you about double, and you'll very likely have to spend some more on lenses, too.