Have the codes read. My truck behaved the same way when my speed sensor went. I had erratic speedo either was not picking up right speed or jumping around. Trans would not shift right. Changed sensor everything was right with the world.
Whats funny, is this exact same thing happens with the locomotives at work with the electric traction motors. With a locomotive though it just disables the motor with the failing speed sensor, but for each motor you loose you loose that much more power to pull the train.
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Exploration Machine- 02' CC V8 swap. 300K+
Portable Roller Coaster- 07' MKV VW GTI
Its another Jeep- 98 TJ