An F250 is more than a bit of overkill for just hauling an ATV... That F100 would be plenty, it's just that 21 year old leaf springs just ain't what they used to be....
My 86 F250 is rated at 8800 LBS GVWR, I could easily put that F100 in the bed and be under that.
4wd is nice, but, if you're up to it, on an 80-83 F100/80-96 F150 it's a bolt in swap on a 2wd truck...
Evan