It's fine. Actually, it's fine on 4WD too if you hardly ever use the 4WD -- we have several work trucks that operate in 2WD 99% of the time and I often replace tires two at a time. My work truck goes off road all. the. time. so it does have AT's all around. My technician's truck rarely does and it has highway tires up front.
Don't waste your money on anything made by Goodyear or Cooper. I have had four Goodyear tires go bad on my truck (two had to be replaced - one at 5000 miles and the other at 8000 miles along with two more that needed to be patched!). Coopers made the ride horrible and the tread wore out way too fast.
My truck has Michelins on it now and what a huge difference. Expensive but well worth it!
"When a horse greets you with a nicker & regards you with a large & liquid eye, the question of where you want to be & what you want to do has been answered."CANTER New England
The trouble with different front and rear is that it limits the options for rotating. I put 115,000 miles on the last set of tires on the dually because I was able to rotate as needed. The front ones wear more on the outter edge of the tread, and the back ones more in the middle. At 115,000 miles total, it's obvious that the wear is not fast enough to warrant any kind of alignment or pressure change, but there is a small difference. There's also a difference on the outter edge on the rear ditch side tire. Not being able to rotate might cut tire life for this truck in half. I don't enjoy spending money for nothing, but have had great service out of Michelin LTX's.
The longest lasting tires on my F-250 4x4 were actually Toyo Open Country tires. I was not happy with the BF Goodrich A/Ts. Wore out fast, didn't give me any traction in mud or damp grass and the truck rode like a "deuce and a half" according to my father. I have Pirelli Scorpion ATRs on right now and the ride is wonderful. I haven't had a chance to really check it out in mud.
F-450 and 550 trucks come from the factory this way. Yes, 4WD.
It is not quite as big of an issue on a 4WD vehicle (vs AWD, where that is a 100% absolute no-no unless you want some EXPENSIVE repairs) where you have the tires on separate axles (ie the two up front can be different than two on the back, but they MUST be the same on each axle) but it's not something I would particularly recommend doing, if only for the issue with rotation that was mentioned above.