I've got a '90 F250 with the 4.9 straight six. It's got 4.10 gears. A couple of important things, during this era, Ford made"light duty" and "heavy duty" F250s that look identical. You need to check the GVWR on the door frame to find out which it is. If I recall, the light duty GVWR is around 6000, whereas the heavy duty is 8600+. If you want to haul heavyish loads, do NOT buy the light duty model. Also, Dr. P is right, Google "Ford Vin decoder". In an ideal world, the truck would have 4.10 gears, and a limited slip rear end. The decoder will tell you what you've got.
I'm very pleased with my F250. Mine's 2wd, which is too bad, but it'll haul a heavy load. I've put 4000# in the bed before (no exaggeration, I could dig up the feed invoice for proof). Since my truck is the absolute base model, it's very light and has a massive payload capacity for a SRW truck. I won't say that it's got a lot of extra power. The 5.0 won't either. For heavy loads in the bed it's no problem at all. For a heavy trailer, you'll be going slow up hills. I towed my 3500# tractor on a relatively heavy skid steer trailer into a strong headwind. I couldn't do 55-60 in overdrive, so I had to pull it in 4th most of the way. Right around 2800 rpm for about 2.5 hours. No complaints from the truck, though.
Good luck with your search.
