After 12 years of continuous Ford Explorer ownership, I was forced into buying a replacement vehicle while on vacation towing a camper, two days before hauling it and the family into the Colorado Rockies: the Ford Explorer's transmission was dying at 120k miles - go figure.
Without much of any planning, we bought a V8 4Runner, threw everything from the Explorer in it, and haven't looked back.
I would do the same again. The torque and towing capability of the V8 Runner are right where I wanted to be (plenty left over), after feeling for many years like the Explorers were just always underpowered.
I have towed about 10k miles plus daily driver miles. I haven't yet experienced any transmission issues, but will be performing all of the recommended maintenance.
Differences you will notice: the Explorer has creature comforts all over vs. the 4Runner. That's one thing the Explorer excels at - from heating/cooling handling, seat comfort, ammenities, etc. The purpose of the 4Runner's design is bent other ways.
Offroading - possible in a 4Runner, not possible in an Explorer. Yes, I could drive over small logs and a few other puny features in the Explorer, but I have far surpassed what was possible using the 4Runner.
The Explorer has much more interior space. I really had to think hard about giving up the built-in third row fold down seating, and other storage possibilities, but am finding work-arounds for some of that.
For towing, the rear of the 4Runner will sag like crazy. When hitching up approximately 325lbs of tongue weight, the Explorers held pretty well. The 4Runner drops several inches with stock suspension (Sport Edition, Xreas). That's one area I'm looking to do a major upgrade in anyway. I do like the larger brakes of the Sport Edition, for stopping power. I'm even able to crawl under the 4Runner without jacking it up or putting it on ramps - nice!
There are a lot of other fine differences you will encounter, especially since you've had as long or longer history with Explorers. Some of these are good, some can be overcome with modifications. Some are just going to be that way, period.
I am looking forward to many, many years with the 4Runner and actually having fun, more than just running to the grocery store and a camper-hauler-only vehicle. Check my sig.