It started about a month ago on the interstate when a car quickly stopped in front of me... I went to downshift to use the exhaust brake, but the transmission wouldn't come out of gear! Needless to say, not a safe situation. When I applied the clutch pedal, it felt like it had a lot more resistance, and it wouldn't disengage the clutch. After pumping the pedal several times, it finally released. As of this week, it's started happening more often... probably occuring during 1 out of every 4 attempted shifts. While pumping the pedal will finally release the clutch, every time it happens, the aroma of burnt clutch disc fills the cab. Has anybody had this problem? Maybe a slave cylinder going bad? Could this be the result of any mods I've made to the truck? Thanks in advance...