AAMCO does good work and has a good warranty . . . provided you're driving a 2WD truck.
They will not offer the lifetime warranty for any 4WD vehicle. Been burned too many times by the "hold my beer" crowd.
Several years ago we had an AWD Chrysler Town & Country van. It developed issues with the front transfer case, CV, steering unit thingy, which required a number of attempts to get repaired right. My wife lost faith in it, and we traded it in for a new Durango in 2004. To be fair, the Durango has been very kind to us from a maintenance point of view (almost 200k miles with no major maint work). However, about a year into the ungodly new car payments, the wife realized that no matter how many times we had to have that front end repaired, there was no way it would have cost us more, and we actually LIKED the van. Best open road car we ever owned.
My advice, if the truck is in good shape otherwise, get the tranny fixed and add AAA to your wallet. Seriously, you don't already have AAA? You know, it only takes a single tow -- especially if it's from the boonies -- for that to pay for itself.
BTW, one of the things that can make your tranny act up is an accumulation of crud, a simple by-product of friction, in some of the pressure channels. AAMCO locally pulled that piece on mine, blew out the crud, cleaned it up with solvent, and reinstalled it as part of the tranny flush a couple of years back, and the thing has run fine since.
Engine ran rough a couple of months ago. Needed spark plugs, of all things (same engine you have).
Another thing that can mess with your power and gas mileage is the fuel injection system. The feed lines can be cleaned or replaced and that can make a real difference.
And we STILL don't have a car payment.
Car payments are evil, and must be destroyed.