I'm a big guy as well, and there is no easy way of saying it, but you need to do pull ups. It may be demoralizing at first, but you have to keep at it. I was the same way, couldn't even do one. Now I can do 15-25 behind my neck at 250. When I get back down to my 220 should be even better.

If you don't like people watching (I know I hate looking weak) get a cheap pull up/dip station for the house off Craigslist. I bought mine from Academy for about $80 so you should be able to pick up a used one for $25 or cheaper. Just keep at it. They normally have a bar you can spot yourself with your leg to help at first. Just set aside 20 minutes every 3 days and do what you can. It's going to feel weird and awkward at first but you'll get it eventually. Just takes time.