Just in case anyone was wondering, I thought I'd leave an update. The trans pump was blown, and the bullgear at the input of the transmission didn't look too hot. We're having the whole thing rebuilt now.
I guess after 140k of hauling an 11,000 lb. service body truck, the trans had just taken all it could. The strong performance of the engine after our injector replacement was probably just the straw that broke the camel's back.
Once again, I appreciate that ya'll tried to help. Have a good one!