No way to know for sure on gearing. the standard gearing in the 8.25 axles was 3.21 with 3.55 and 3.92 being options, along with optional LSD available in all gear ratios.

however, being a V8 truck you PROBABLY have the 9.25 axle, and I've not come across a 9.25 with 3.21s, so chances are, it has 3.55 gears, with 3.92 being an option available. Again, LSD was available as an option in both ratios.

Dakotas have a sticker inside the glove box door that has the paint code, some trim codes, but it will also have the rear axle size, and gear ratio, as long as nothing has been changed from stock.

being a used, 13 year old truck, how knows if a prior owner swapped gear ratios, added a LSD, locker, etc. (my toy Dakota now has 4.56 gears )