98 Silverado tranny problems??
 

I've got a 98 Silverado with a 5.7L. Leaving a stop, getting up to right around 35 or so, truck shakes a little bit, so I put it in first (Its automatic), and nothing wrong, second nothing wrong, but then put it into 3rd and got to around 35 and it started shaking. Would it be the tranny, or the rear end? I have noticed a slight humming noise comming from center or rear of truck while cruising down freeway at speeds. Its kind of difficult to hear well because pipes are loud. Or might it be the driveshaft? Can I "wiggle" the driveshaft to see if its bad? Any comments or info would be greatly appreaciated. Also how much it may cost for parts. I will most likely do it myself. THANKS!

to eliminate the driveshaft and tires...drive at the problem speed (35 mph) and then go just above that speed (40 ish) and shift into neutral and let the truck come back down to the trouble speed (35)...if the vibration is stil there then it is either a u joint,tire or front end problem....if it is gone...then look at a 3rd gear shutter or a possible bad tranny mount, could be the tranny mount either way.

Thanks