Your 48 1/2-ton rear end has a 4.11 (or 4.10) ratio. The 55 is 3.90. Is it from a 55-1st or 2nd series pickup? I am real sure that the 55-1st will be a direct swap into your 48. Spring perches will line up, etc. Same for same, good swap for that truck. The 55-2nd and later (thru 59?) may be just as easy. Do some measuring: spring centers, overall width, etc. to be sure. You'll have to get or make a correct drive shaft. Good luck.
