I have an E150 and once installed 30x9.50 Firestone AT tires which were 29.5" tall. They fit and did not hit the wheelwells in the month that I ran them. I lost fully 2 mpg over Michelin LTX M/S tires in the stock size and opted to return the Firestones within their 30 day refund period (I ate the cost of mounting/balancing which was far less than the extra fuel). The MPG loss was after adjusting for the extra diameter. The standard upsize for 235/75 has always been 255/70 but those do not give you extra ride height. In your case I would go with 30x9.5's.
You could probably fit 31x10.50's on the back with 30x9.50's on the front, but the larger rear diameter would mess up the gearing even worse.
A 265/75 would be 30.65" tall by my calculations...do you mean a 265/70x15 at 29.65? The latter would probably fit the rear but I would be concerned about the front wheel wells when turning the wheel--the tire tread edges would stick out pretty far and that is getting to be a really wide tire for a stock 7" rim. If you get different rims, offset becomes a big issue.
Finally, even the 30x9.50 tire added enough drag that my 4.6 had to work much harder on uphills that I am familiar with, downshift to 3rd on hills that the van used to take in 4th. If you have a 4.2 or 4.6, a tire diameter increase may "overgear" the van slightly. This would be part of the mpg loss I saw.
2000, E-150 with 5.4L here. I ran 31/10.5/15 BFG AT on the stock alloy rims and it had a slight rub on the sway bar at full lock. No problems with stability, a little noisy, good on snow but not in heavy rain.
Now I run 265/65/17 Cooper Discoverer AT3 (30.5") on 17x8 rims 0 offset. I had to remove the plastic cowling under the front bumper (It could have been trimmed) and trim the rear "seam" on the back of the front fenders. After that no rubbing. I love this tires, fairly quiet and very stable and responsive. I could cruise at 90 mph all day. New springs and Bilsteins on all four corners.
Good luck with the tires. I have put 255/70's on 4 different vans and a pickup as the "standard" upsize from a 235/75. Sticking with 235/75 Michelin LTX M/S2's on my current van. The 265's will definitely fit in the rear but I would watch for the tread edges to clear the wheelwells when the tires are turned and you hit a big bump. This is gonna happen if you are off road and the sounds of tires meeting sheetmetal are not good. (I had BIG interference on my ancient '73 E200 and just a "little" on my '86 GMC 2500 with 255/70's.)
The 30x9.50's are ~the same height as the 265/70's and I have used those with no clearance problems. I'm guessing the 265/70's are available in P-rated tires and may give a better ride and gas mileage than the LT flotation 30x9.50 tires.
Also if you want to go off road, put a limited slip axle in your van if it does not have one. I used to do fire roads with the old '86 GMC and it would get thru a LOT with an old Auburn limited slip. I have an Eaton in my E150, installed when the van was new.