Re: fear of ukemi
I weigh around the same as you (266 lbs ish) and i'm fairly tall (6' 3") ukemi is one of my big big aiki worries. Like you I can be surprisingly agile or a guy of my size but there is still this knowledge that I am this size and if something goes wonky in the roll then something is going to break!
My main concern is that I am a big pussy! I can roll to a reasonable degree on my left (reasonable degree meaning that i am 'safe') but on the right it's a different story. I used to think it's because i'm right handed and i want that hand free / don't want to injure it but now i think the problem lies in my legs (my right leg is weaker) and i can't get into a starting position where i feel comfortable to begin the roll.
I spend a lot of time rolling from kneeling: doing the 'baby' style where you reach back to touch your toes in seiza/kisa, the slightly higher (usually one knee down, one knee up-think proposal of marriage) then in a crouched position (ape-man!)... Using the arms as a guide, sometimes trying to "miss out the arm" to help me realise that my arm does nothing only gives slight help in guiding the movement, sometimes break falling sometimes doing it "properly" (standing up at the end).
I'd advise all larger peeps to learn how to break fall... in fact, all people should know even if they never have to!