There is a vibration in my 1998 Chevy S10, 4 cyl., 5 transmission truck when I shift into 3rd gear. The vibration lasts until I reach about 38 MPH and then stops. There is also a clunking sound when I shift gears.

Sticking valves. When is the last time you had the transmission serviced? Since it will shift when cold, I would suspect a valve body issue.

You don't give me much information to work with. However, 1st, look at the engine mounts. You may have a broken mount. This will make it feel like the transmission is shifting hard. Other than that, there is only a few things that will cause this. The speed sensor tells the computer when to shift the shift solenoids. It may not be "seeing" a quality signal. If this is the case, the speedometer will be bouncing too. Last idea is that it is skipping a gear. Starts in 1st, should go to 2nd at about 10 MPH. 3rd at 25 to 30 MPH. And, if you have a Overdrive transmission, it should go into overdrive at around 45 to 55 MPH. If it is skipping 2nd, it will engage 3rd at a high engine speed, and will feel like a sudden slam into gear. If this is true, your transmission needs some work. Hope this all helps. Good Luck.

shift solenoid for third gear maybe bad up inside the transmission pan

transmission clutches bad for third gear operation. needs trans overhaul..

replace transmission filter!

Hi:

Your transmission has 3 shift rails...
1st - 2nd
3rd -4th
5th- Reverse

If your 3rd gear doesn't work , you have an internal transmission problem, such as a syncronizer, or similar fail... There is not much you can fix, without removeing the transmission.

Sounds like the passing gear cable is out of adjustment. Get a shop manual and see what the specs are on how to adjust it properly. It is a pretty simple fix.

Sounds like you may have accidentally removed the vacuum line to the modulator valve on the back of the transmission. (worst case, the headers may have burned through the hose, but either way, this is more than likely the problem.
Hope this helps
Chris.

The transmission/computer is designed not to go into overdrive until it reaches a certain temp and I don't remember what the temp is. Mine does same thing and it has 250,000 miles on it. When you start your truck up do you start it and warm up in neutral? The transmission fluid won't flow unless it is. There is know way to force it to shift into overdrive until it warms to the set temp.

Hope this helps
Howard

one unit that operates ratio changes and is connected to the engine is that vacuum operated actuator on the transmission
it is possible that that unit is failing , vacuum hose is leaking and that will cause ratio changes as the vacuum varies
run the fault codes to check for wiring problems or VSS or TCM problems as well