Mark your rear tire sidewall where the tire meets the ground vertically, then tie a string around your driveshaft and knot the string at the bottom of the driveshaft and mark the driveshaft with a grease pencil or welding crayon. Now have somebody get in your truck and move it slowly for one tire revolution while counting the driveshaft revolutions with the string. If it's a little over 4 revolutions on the driveshaft, it's 4.10s. About 3 and a half, 3.55s. If it's a little over three, you've got 3.23s. If you got lots of time on your hands and like the smell of gear lube on your hands (or maybe you just haven't changed the lube and you want to do the maintenance), pull the rearend cover and count the number of ring gear teeth and divide by the number of pinion teeth. Just wondering: how tall are your tires? It will effect your final gearing.
In regards to your power or lack of power: timing is pretty critical on an IDI, so get it timed. If it still runs poorly, look at your IP and injectors.
Good luck, and FYI: that's a good looking rig in your avatar