The 4x4 crossmembers are the same for 73~84 1/2, and 3/4 ton. 77~84 1ton is different, and specific to that frame.
85~91 1/2 and 3/4 ton are also interchangeable, whereas the 85~91 t-ton uses a specific crossmember as well.
The 1-ton 4x4 frame is about 1 3/8" taller, and that is why it uses a different crossmember.
The older crossmembers will interchange with the newer frames, and vice-versa, but you have to use the corresponding model years tranny/tcase mount that matches said crossmember in order for things to work out right.
I.e. you must use a 73~84 tranny/tcase mount with the 73~84 crossmember, and the 85~91 tranny/tcase mount on the 85~91 crossmember in order for things to work. What year frame you use it on doesn't matter.
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