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[guardrail22]

three observations

12000# is very near the limit for the F250 so you are at the outer edge of capability on things like brakes etc.

an exhaust brake retards the engine speed to create braking effect. on a std transmission, this is effective because with clutch fully engaged, the drive line is solid from crankshaft to wheels.

for an exhaust brake to work on an automatic where there is normally NOT a solid driveline because of the torque converter, the brake is only going to work if its control features include a torque converter lockup.

This can be done by using tow haul mode on the Torqshift transmission.

I'd first have the trans and PCM checked to insure tow/haul is working. Exh brakes are a $1000-1200 deal plus install. I looked into it briefly and decided against it because I couldn't buy just the brake (alrady have 10gal of 125psi air on board).