Truck running rough and exhaust note changed, where to look?

[Sworn]

I could really use some help as my truck has started to run rough. I recently had the exhaust system replaced as most of it was rusted out. (The truck is twenty years old and from the Midwest rust belt.) I put on a new cat converter with a Flowmaster muffler. The truck ran and sounded great. I was so pleased with how it was running I took it to a trans shop to finally have the trans pan gasket replaced. The trans pan had been leaking for a while and I finally wanted to get it fixed.

Now here's where the problem starts....The minute I started it up and pulled it out of the trans shop I noticed it was running rough. The engine had a slight shake to it and the exhaust note completely changed. it also has a notable loss of power.....

My only conclusion was that something became jarred when they were working on it. I took it to my muffler shop to check for exhaust leaks but was met with negative results.....I also checked for vacuum leaks, once again, nothing. I went out and bought an OBD1 scanner and found no codes.

I am completely stumped and could use some advice! Thanks in advance!

BTW I tried the "slightly block the tailpipe test" and noticed no exhaust leak....

 

[Hahnsb2]

For whats it's worth I've noticed my exhaust note changes with different ignition timing, maybe check that?

 

[CraigK]

Just a suggestion, check the vacuum hose that goes back and down to the Auto Trans modulator valve (if you have one). It T's off at the back of the engine. Vacuum hose could have come off at the transmission end during transmission servicing.

CraigK

 

[Sworn]

Just a suggestion, check the vacuum hose that goes back and down to the Auto Trans modulator valve (if you have one). It T's off at the back of the engine. Vacuum hose could have come off at the transmission end during transmission servicing.

Didn't check that one. Thanks.

 

[Sworn]

Just checked the vacuum hose to the trans....it was connected and seems to be in proper order.