Can anyone tell me what the fork diameter is on an 82 XJ750?

Less than 42 mm ;)

http://www.oldbikebarn.com/motorcycle_part...eals&bike=XJ750

Depends what year - from 80 through 84 they were one size, in 85 the size went up it seems. Reading from what I can on the seals page they were 36 and went to 38mm.

I'd check your manual as mine is at home and I'm at a building site...