Symptoms:
Whenever i slam on the gas, i get a loud thump and i'm planted in my back seat. Also, transmission slips all the time in 4th gear OD. 111,000 miles on the tranny, how much for a rebuild approx.?

 

Oh yeah... also, my Posi went out if that has to do with anything =(

 

If you hear a thump when you accelerate check the crossmember on the tranny. The cost of a rebuild depends on where you go and what parts you get. I would think a shop would charge around $1500, but your best bet would be to find a local mechanic. I got my tranny rebuild with a used TC for $700 and that was for everything.

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'86 IROC
T-TOPS, TINTED WINDOWS, BRAKE LIGHT BLACKOUTS
GM GOODWRENCH 350
EDELBROCK TES HEADERS
FLOWMASTER, NO CAT
EDELBROCK 600CFM CARB.
KN AIRFILTER
ACCEL HEI DISTRIBUTOR
160* Stat
3:73 Posi
Rebuilt 700R4 vette servo and .500 boost valve
B&M Megashifter, 5" Autometer Tach w/shift lite
Soon to be installed
3" Hooker Aero Chamber

 

Oh and welcome aboard,
This site will really help you with your troubles.

 

the loud thump is probably your trans mount. The GM and part store replacements don't last very long. Spend a little more and order one from Summit or Jegs, polyurethane mount, these hold up to a lot of power and last for many years-- slipping in 4th gear I could not tell you what to do