When I start in drive from a stop, it feels like my brakes are locking up but I think it is my transmission. How can I fix this.
If it is the transmission, it will need to be removed from the truck and opened up to find the fault. There is no other way to get to the internals, like shafts and bearings, without pulling the transmission. This goes for automatics and manuals.
I’d make sure that it is not a brake, wheel bearing, rear axle, or driveshaft problem before condemning the transmission.
Is your truck a K20, i.e., 4 wheel drive?
We need more info on the vehicle, like what year and engine size, 4WD? I need to know what transmission we are working with.