I have a 94 full size Blazer. I get a bit of a thunk when she moves ino third gear and the engine seams to wanna shut down. It studders a bit, similar to when the timing is off with the engine and distributor cap. However, the cables from the spark plugs and distributor cap are only 15 months old, as well as the plugs themselves. The engine does have 182K miles on her, so wondering if she's on last legs.

So my question is what's going on with my transmission and is it possible that I need to replace my distributor cap, or is that engine issue another problem?

 

I'm guessing it's speed related, but it very well could be the transmission. I would start by watching RPMs as well as vehicle speed. Put shifter into D or 3rd so you are not in OD. Try it and see what happens. The transmission operates differently in 3rd gear if Range selection is in D or OD.

Does it make the noise/engine problem every time?
If you put the rear up on stands and run it will it make the same noise/symptoms without the load of the vehicles weight? Use caution and lots of common sense if you do try it.

I would also check the transmission mount. Strange stuff can happen when those are worn out.

Do some investigating and maybe something will turn up. But start with the easy stuff and see if anything changes.