I have a 93 with the 5 speed mazda transmission and a 4.9l. When I drive the truck a vibration begins coming on around 3rd gear and gets worse with speed. It only does it under load, if I press the clutch in the vibration goes away and all is smooth. It has gotten progressively worse. I looked into the bell housing and saw that the slave cylinder is wet but I don't have any of the typical hard to get in or out of gear problems. I did have to add some fluid to the clutch about 2 months ago and haven't had to since. When the vibration first started I was due for a tune up anyway and also replaced a leaking water pump. Looking at the clutch it looks like it has a good bit of life left to it. I changed all of those transmission components about 10 years ago and over the past 10 years may have put 30-50k miles on the new clutch, slave cylinder, pilot bearing, rear main, pressure plate. I have searched the forum and really haven't found any definitive answer on this.

Any help is appreciated

 

sounds to me like you have a bad ujoint, take a look at your year drive line, pull it off and replace your ujoints and relube it and I bet that will fix your issue.

 

As for the fluid on the slave cylinder, it COULD be that the connection lines have worn loose, and given it a slow leak. As for the potential u joint, if that is the case (and it sounds like it to me), and it's left unchecked, it will tear your output seal on the transmission up, and you'll have a big problem. Another less likely thing it could be is that since the driveshaft is balanced with weights, some of the adhesive holding the weights could have worn out, and you might be missing weights. You'll be able to tell that if you look where they are at, and you'll see the outline of the missing weights. No outline = no missing weights. The U-joints are the most likely issue though, as it's rare for a weight to come off there, and you would have likely heard it hit something unless they got slung downward.

 

x2 for ujoints. Took me a bent shaft to learn that lesson.

 

I'll plan on changing the u joints this weekend. I looked at them and the drive shaft didn't budge when I shook the truck back in forth in gear. They are cheap enough and I know it can use them anyway. Will let you know the outcome of that.

 

Spent today with my brother changing my clutch, pilot bearing, rear main, slave cylinder, u joints and oil pan gasket. Turns out it was the clutch. I figured that would be it after driving it a few more times. The old clutch had separated a bit. It had plenty of wear left on it but where the transmission shaft went through it had some play in it front to back but was still attached. Weird. The oil pan gasket was a B@#ch. Just would not cooperate.

Just wanted to follow up and let you all know. Thanks for the responses.
Walt