First off, don't buy basking bulbs at a pet store. Flood/spot lights from any hardware store will fill the same need, while costing significantly less. Often times they are the same bulbs... (remember, I'm not talking about UV bulbs here).
Second, how are you measuring temps? Are you looking at those dials on the wall of the viv? If so, I wouldn't trust what your reading. First off, the dial gauges can be pretty inaccurate, second the basking spot temps should be measured where the dragon is sitting, not the wall next to it. If its 95 degrees at the wall, chances are the basking spot is a bit warmer as the light will be shining directly on it. If you don't already have one, a digital probe thermometer will be a worthwhile investment as a beardy owner...and they are cheap -
http://www.reptilesupply.com/product.php?products_id=1714. Should be less then $10 at any local pet store. If you did measure the basking spot temps with a digital probe or IR temp gun, then read below for my advice on how to raise basking spot temps.
If you can't raise the basking spot any higher, and you can't lower the bulb, then you're pretty much stuck. You said you have the bulb hooked up to a dimmer? Is it adjusted to full power? 150w is a pretty powerful bulb, I find it hard to believe that it can't keep the basking spot warm. Though after looking at the tank, it appears to be rather tall...so that probably explains it.
Can you make a second story somehow? Perhaps an homemade basking platform is in order, something that will raise the basking spot higher? You could make something out of foam and grout, or you could stack up some bricks or other stone pieces.
The only other thing I can think of is to A. Lower the bulb somehow, or B. go and get a spot light (not a flood light)...spot lights will give you a tighter beam of light and heat, and should penetrate farther into the tank. Using a halogen bulb will help as they offer more light and heat for a specific wattage (compared to incandescent bulbs). You could also add a second bulb, but that will require a second light fixture.
A 100w halogen spotlight will cost around $7 at Home Depot. That should be more then enough bulb to get a basking spot above 105. At anything less then 3ft, I'd expect you'd have to dim the bulb significantly...as they put out a LOT of heat. I have a 50w halogen spot light, and at full power it can easily heat a spot 2ft away to over 120 degrees...I have to keep it on a dimmer.
Can you take some pictures of the full lighting setup?