High miler 98 with bad tranny shutter

i've had a few people tell me that the 4 speed sticks in O/D causing this shuttering I am getting. What happens is if the rpm's are up at highway speed and I let off the gas (rpm's fall to coast) The tranny will vibrate and shutter pretty hard when i lightly press the gas again....All i have to do is press on the gas harder and it downshifts out of it (rmp's kick up)...also noticable when accelerating on a hill. Tranny also shifts 1-2 kind of hard. Fluid is pretty clean and at the proper level. Car has about 180,000 miles on it. I read its good to shift 1-2-3 manually instead of selecting Drive, any truth to this? It seems to not shutter when shifting manually. Just got the car and the SES + engine lighs are on so Im gonna pull those codes when its getting looked over for safety....Any ideas???

thanks for any input

Welcome to AF.

Best thing to do is get it scanned with a trans capable scanner and see if any tranny DTCs are stored. From your symptoms it sounds like you may have a torque convertor clutch (TCC) or have the classic bad 4th clutch shaft problem. The shaft splines strip out or shear. Symptoms of 4th shaft problem is a shudder when shifting from 3rd gear to 4th (OD), a loss of 4th completely and accompanying symptoms of where it drops to neutral or coasting. With your mileage it sounds like rebuild or replacement time.

LubeGard sells an additive that helps to free up any stuck TCC solenoid. I had shudder on taking off from a stop and it definite help the transmission shift better. At $10, I would try that first. It comes in a red bottle.

well, im the 2nd owner. The first owners were old people who drove to florida ALOT...thats where most of the miles came from. Tranny fluid was changed a few times in its life (3 times I think....once in 2000, again in 2002 and last summer 2006 according to the service schedule.) Im gonna get the codes out of the ECM this week, maybe try a tranny additive. Lets say I dropped this off to get a 4th gear clutch fixed, what $$$ am I looking at on average?

pulled 2 codes....mechanic said misfire cyl #1 and some emission evap code (likely due to gas cap vent)...im seriously starting to think its a misfire...new plugs to start!.....BTW it does not look fun at all to get at those back spark plugs.....are they a real pain in the ass? any tips? what plugs do i need?

Based on the misfire DTC go with spark plugs and possibly new ignition wires first due to the mileage. You can use AC Delco, NGK or Autolite Double Platinums or Iridium. The current recommended AC Delco replacement spark plug for your year and engine is the AC Delco Iridium 41-101. The original were AC Delco 41-921 or NGK PTR4B-15 which are double platinum.

With the right tools and patience the rear plugs are DIY. Some people remove the front dogbone engine mounts and move the engine forward but that is not necessary. Normally the worse part is removing the ignition wire boots off the spark plugs.

On the evaporative emissions DTC post it if you have it since there are several. Always start with checking for a loose or bad gas cap. Good luck.