300+ here, and loving a Thudbuster LT purchased on sale at Amazon for about 100 bucks. Truth be known, Amazon often sells returned items that are brand new with no or open original packing, as "used". I bought one for 90. I think, but they sent the wrong size. When informed, they sent another, unopened seat post with a 10% discount for my trouble (a little over 80. total)! The best part is Thudbusters are tunable using different cylinders made of progressively harder material. It shipped with a #7 and a couple others. I ordered an optional #9 that I used with a 7 and it works VERY nicely! It actually feels like my rear tire is flat!
With the fatty tires, I would suggest you ride the Rover a bit to see if you still think you need a suspension seat post. -Al