I recently sold my 2006 when we traded in the 22' TT for a Class C. It had 101,000 trouble free miles, nothing but brakes shocks and tires over nearly 5 years of serious towing through the mountains of the western US. By all means get the plugs replaced, I paid the local shop to do it at 72,000. I went with Monroe LT lifetime warranty shocks and that, along with a set of Michelin M/S 2 LTX tires dramatically improved the ride, handling and overall towing experience. Replaced the brakes at about 75,000 miles and they were fine when I sold it.
Had we not shifted gears and gone with a nice C I would probably have kept the truck for another 10 years, hands down the best vehicle I ever owned. The 5.4L is an awesome motor that delivered not only excellent power but decent mileage to boot both towing and non. I would recommend the OP spend a little and keep it, especially given the ridiculous prices for something newer. Yes I miss my truck but could not justify keeping it with nothing to tow. :C