I am 6' 3" Have had a bad back forever even with surgery in 1996. My back has gotten worse after a period of being better after surgery.
For me, like many say, it's different with everyone, it was finding the old conversion vans, specially Chevy G20 conversion van. I like the ones that ended in 1995, but I am pretty sure they continued on with the enterior design for a few years. Of course the new vans are very different and I cannot speak to those.
The thing I know that works about them is that the position is not unsimilar to a chair, meaning that my legs are not having to go nearly straight our towards the pedals, but gently bend just enough to not pull at the chain of muscles. Also the seats allow me to sit up and keep my back straight and comfortable.
I will say that I recently (2015 actually) rented a 2015 Tahoe and it was so awesome comfortable that I drove from Ventura County to Carson City, NV in one day and without any real pain. Any car I have driven, including Volvo, Toyota Camry, and even the large Crown Victoria Fords gave me real problems.
So, I now drive a 1995 Chevy Van (G20) conversion van and have very little issues. I would have to say the only other vehicle which may be even better are the newer Tahoes and Yukons. Not sure about the Suburbans, but I would think they may qualify.
I feel your pain and though this is many years later, I hope you have discovered what works for you. I know I have finally found that the '95 Chevy Van and the tahoes and Yukons (late models for sure, not sure about the older ones, as I have not tried them out).
My personal experience in looking for and finding what works for me may be a generic prescription for you as well, because the trick is to have a seating situation that does not allow for the pulling of the lower back muscles over time which will tork you out of shape and into the pain zone. I was shocked I could actually drive 4 hours in the Tahoe, much less the 9 or 10 hour trip I ended up making, For that I felt wow how awesome I could feel normal again!
Good luck with your search and keep those legs bent on those long trips. Oh how I miss my old english sport cars. LOL, in those, especially my '73 mgb had so freaking much leg room, but I was young then and my legs could stretch completely out and and would have to stretch out to fully depress the clutch (36" inseam). I can no longer drive something like that, but felt inclined to mention it. LOL