Re: Install new flexplate myself or take it to a shop
First of all, you have to remove the transmission and transfer case to replace the flexplate. It's not hard if you have the right equipment. The easiest way I've found is to remove the transmission tunnel cover and use an engine cherry picker to lower the trans/case after unbolted. A 4' long chain is needed. Be sure to disconnect all bell housing bolts, front & rear driveshafts, speedo cable, trans cooler lines, shift linkage, cross member bolts, electrical plugs (if equipped), etc. before removing the trans. Be sure to remove the 3 flexplate torque converter bolts and remove the torque converter with the transmission (or you'll have a mess!). There's 6 bolts that hold the flex plate on. R&R the plate and torque to spec... then replace the transmission.
It's really pretty easy, if you have the equipment and general knowledge of how to do the job. Do yourself a favor and don't drop the tranny on yourself or your buddy trying to lower it by hand. A TH400/NP205 combo has to weigh somewhere between 300-400 pounds (if not more) and it's one awkward bubba when it's in the air and not bolted down.
I'd expect almost $1000 to have a shop do the job. Costs you the price of the flex plate, a couple hours work, and the cost of a case of beer to do it yourself.