Loud metal grinding noise when starting from a stop and trouble getting truck into reverse, it's like it's locking up.
My car has 150000 miles.
My car has an automatic transmission.
The first task would be to determine, based on whether you have a two- or four-wheel drive vehicle, where exactly the noise is coming from. If you have four-wheel drive, there are a number of possible sources other than the transmission (e.g., the transfer case, the front driveshaft and differential, etc.). There are also several different transmissions used in trucks depending on whether you have two- or four-wheel drive and which type of engine you have. Unfortunately, you don’t specify whether your truck is two-wheel drive or four-wheel drive, or what type of engine you have; more information would help us identify your problem. In the meantime, I would recommend having a professional mechanic, such as one from YourMechanic, diagnose this grinding sound.