Well, it doesn't look like anyone is going to answer this, sooooo
15x5.5=3.5inches of backspacing
15x8=4 inches of backspacing. (most likely on the pre81 CJ's)
In your case, a set of widetrack axles would be the best (IMO)
OR a set of 15 x 10 rims with 4 to 4.5 inches BS. if you want to run wide tires. It's really up to you.
1975 CJ-6 1983 CJ-8
1986 CJ-7 Laredo 1986 TJ-7 Trail Jeep
2003 Inca Gold TJ Rubicon