Looking for some advise on a truck issue. I have a 89 F-150, 302 Automatic. The problem being this. I replaced nearly every sespension part including u-joints etc. When the truck shifts in third gear, it trembles really bad. Almost like a driveshaft shake. The drive shaft isnt bent or anything else I can see. It does smoothen out after going over 80 kms /hr. Like I said, its around the 60 - 80 kms/hr mark. Any ideas?

Hmm, which auto trans do you have?
Also, have you checked for a bent axle on the rear end?
I kinda want to say torque converter clutch on it's way out, but, without knowing which trans you have, I can't say that with any amount of certainty.

It has a auomatic overdrive. The overdrive is on the stick. Their is no override for the od.

well, youve got the aode then. im thinkin maybe the lockup torque converter? not really sure, cause ive never messed with the electronic aod.

Does it only do it under power (load) or will it do it at that speed no matter what, under power, coasting, or otherwise?