ifs is better than solid axle in some respects, ie stock road manners, but a stock solid axle is better in articulation and rock crawling situations....
the ifs came out in 86 and was present thereafter
the solid axle was in 85 and earleir trucks
if the build date is usually later than midyear, then it is usually the next year model, if it is the beginning to the middle it is usually that year model....
my 84 has a build date of jan 84, my 85 has a build date of may 85