I have had a 3.9L Dakota since last March and it seems gutless as all hell. However, I am realizing that the transmission may be the actual parasite for the power. Does the V8 318 have the same trans as the V6? The Haynes manual says nothing about it. I am thinking that a transmission rebuild may solve my problem. So who has a high mileage 3.9L and how did you get it there? So far I have 125k and it is totally gutless. I use a K&N oil filter, Castrol 5W-30 Syntec oil, Mobil synthetic 75W-140 in front and rear diffs.

not I, but I hear they can go just as far as a 318... like 200,000+ miles

the K&N oil filter and synthetics are a good thing to have on yours

my dad has a 203k mile 3.9 dakota, and it has gradually lost a lot of power. as for durability, i think it would be running perfectly if we had not accidentally run it without oil pressure on a few occasions. it still makes pressure just fine under 95% of circumstances, but clearly would be better if that had never happened. The timing chain rattles like crazy, started pretty much right after i replaced it. theres a tensioner but the noise doesnt hurt anything and i dont care enough to pull the front end off the motor.

as for the transmission, no, the 3.9 and 5.2/5.9 use different transmissions. the v6 use the a500 and the v6s use the a518. however, the design is almost identical, and the 518 is just the same transmission with bigger parts, the the 500 smaller, vice versa.
If you have a 518 and swap to a 500 you may notice a significant difference (look around for people commenting on switching from 727s to 904/999,as these 4-spds are based on those and the same principle applies), but if you already have a 500, its unlikely the transmission is doing something wrong that is sucking down power. if it is not slipping, it is not sucking down more power than normal. however, the torque converter could be. a good way to check would be to install a higher-stall torque converter and see if it gets better, hehehehe. also, see what rear gear you have. if for some reason it is below 3.5, thats your problem. a lot of v6 owners go to 4.10s but then gas mileage will never reach 20 again. there's various things you can do to make a 3.9 truck faster, but to make it FEEL more powerful,here's my suggestions.

listed in order of difficulty/expense.

1. underdrive/lightweight pulleys. sometimes they come up on ebay for cheap, any 3.9/5.2/5.9 set should work.
2. Higher-stall torque converter. if you pay someone to do it expect 800-1000 dollars. do it yourself and save the 300-400 labor. PM me if you want to try this.
3.Lower rear gears i.e. 4.10s. parts will be a few hundred and this is a great time to install a limited slip if you dont have one, but thats hundreds more.
3a. Shorter/lighter wheel/tire combo. Shorter tires change your gear ratio just like a different ring and pinion would, but this one is really dependant on how you you want your truck to look.. i.e. i think 26" tires might look ok on a stock height 2g dakota, but i dont know what you have/want, and obviously short tires arent compatible with some trucks/looks.

Most other mods will make you HP but not give you the sensation of torque like those will.