Selonoid? or Tranny?

[reecechris26209]

Tranny went out. 1999 7.3L powerstroke f-250. Had a mechanic put in a jasper. Slammed into gear after he had installed it. I took a long trip. did ok then lost overdrive, high and low gears. Got it home, spewing transmission fluid out near transfer case. Mechanic said he put it through a stress test and found 3 of 5 selonoids firing. Also said tranny fluid leaking because overdrive was out and fluid was getting too hot. Does any of this sound right or am I getting screwed. Paid the guy $800 for the tranny job when everyone tells me after the fact that it was only a $400 job. Completely confused about the selonoid explanation and what it would have to do with losing gears. HELP!