The square body Dakota (88-96) track width is more narrow then a full sized pickup of the same era by about 6". The rear axle itself was what made the 88-96 Dakota more narrow and the 1" -2" wheel offset made it more pronounced). The early 8 1/4 Dakota axle failed from a healthy small block, so back then several guys put a full sized pickup axle under the Dakotas and the rear tires were at the outer edge of the Dakota bed sides using the Dakota wheels. The front wheel tracking on most trucks pretty much matches the rear tracking width. Wheel mounting surfaces on an 88-96 Dakota front disc was really close to 60" wide, full sized trucks were about 66" wide.
The 6 bolt Dakota axles were about the same width as the older versions, but listed a slightly wider track width. Most of that was because of the 6 bolt wheel offset placed the wheel centers closer to the back side of the wheel rim (very close to being centered). The early Dakota wheels had the wheel center closer to the outside of the wheel rim (more rim behind the center).
Depending on which 9 1/4 rear you have, it could be the older 6 bolt wheels, or the more modern Dakota 5 bolt wheels that I believe have a different bolt pattern then your 75 D 100 has, if that is going to matter. The late 70s "C & B: body 9 1/4 is the same width as your 76 D 100 and probably has the same bolt pattern, but likely has 2:76 gears, with or without a limited slip.
Can't offer any info after the Dakota went back to the 5 bolt wheels. The newer Dakotas may well have used the full sized truck axles, but I think those are 5 on 5" or 5 on 5 1/2" wheel bolt pattern.
