255/70-16 tire size is calibrated for 4:10 rear axles. 235/70-16 for 3:73 gearing. Larger diameter will work fine, but read 60 MPH when actual speed is 62.3 MPH. (Use Tire Comparison and see Speedometer Error)
The best tire size calculator. Use our tire size calculator to find tire size specs and suggested tire sizes. Then check prices for all available tire sizes.
tiresize.com
as @blert mentioned, many things contribute to ride quality and handling. If you are riding on OEM shocks and over 50,000 miles they are way overdue for replacement. I've been using Monroe OE Spectrum (formerly Sensatrac) shocks for years and are comfortable riding but not designed for performance handling. What shocks are installed now?
Tires affect ride quality a great deal. Highway or touring passenger (P rated) tires will "usually" ride much softer than all terrain (LT rated) tires. Also, tire pressure makes a big difference. Unless you're heavily loaded I would run no higher pressure than 32 PSI to soften the ride.
More than ten years and dry rot beginning to show it was time for new shoes. My previous Michelin Latitude Tour model was discontinued so I needed to do some research for replacements. When I bought the Latitude Tours in 2010, I was also considering the LTX M/S2 that was very highly regarded...
sporttrac.org
Body mounts are a tough one. Almost all 2001-05 Sport Tracs have issues with the "A" and "B" pillar FOAM body mounts dry rotting. Problem with OEM Ford replacements is they are discontinued and stupid expensive when you find new old stock (NOS) on eBay. Prothane 6116 or Daystar KF04015 polyurethane kits are good aftermarket replacements but after I recently replaced my A and B pillar mounts found them to be considerably firmer riding. Dorman offers rubber mounts complete with metal shells but they are more expensive.
@caddytrac
Post more details about your truck and any suspension work that has been done.