I just got a 96 V8 4x4 Dakota here today, so I did some looking.
A 90 V6 5 speed 2wd, standard cab, small tunnel
A 93 V6 auto 4x4, standard cab, large tunnel.
A 96 V6 auto 2wd, club cab, small tunnel.
A 96 V8 auto 4x4 club cab, large tunnel.

The difference between the large tunnel and the small tunnel is in both the height of the tunnel ans the width of the tunnel. The 4x4s also have a shifter through the floor pan. I didn't measure the differences, but its pretty plain to see. I suspect the small tunnel will be pretty tight over top of the 4x4 trans & transfer case.

This 96 4x4 I just got has a frame rot problem and will be a drive train, wiring, and computer, donor. This cab looks to have good floor pans from under the truck, but the rockers and the bottom of the doors are shot, and the paint on the roof is pretty well non-existent. If you have an interest in this cab, shoot me a PM. By spring it will be headed towards the scrap yard. Gene