The tranny is in hold mode, where it starts in 2nd gear, upshifts early, and resists early downshifts, all of which will feel like a lack of power. The shade tree mech put an incompatible tranny in the truck, and you are screwed, since you didn't listen to the advice above and get the mech to rebuild the proper tranny.

You can either spend the bucks to get somebody to wire up a tranny mode switch to match the tranny, which will give you a hold mode, a standard mode, and a power mode shift pattern.

Not responsible for advice not taken...