I have a 89 4wd Toyota pickup. As soon as I accelerate over 50 mph my truck starts to vibrate a lot. This happens only in gear and when I am accelerating. When I'm going over 50 and not accelerating or I'm not in gear it's normal. need help, freaking me out. Thanks!

 

The first thing to check is your driveshaft, u-joints, slip yolks for play. Also check your pinions and output shafts for play. Pinion angle could be involved also, your pinion will "climb" under acceleration, and if your angles aren't right to start with it will cause a driveshaft vibration. Worn motor mounts and/or tranny mounts can have the same effect.
is your truck lifted?

 

Thank you for replying. I will take a look under my truck for any play. Could it be my front shocks? When I bought it the guy who previously owned it had oversized shocks in the front, almost about to bottom out when the truck is just sitting. I'm gonna get new shocks hopefully today. But I don't think that would be the issue just because it's only when I'm accelerating. And my truck is on 2 inch blocks with 33's

 

Shocks don't cause vibrations, if you have a bent wheel or out of balance wheel/tire a worn out shock will allow it to vibrate/bounce more. but do not replace shocks hoping to fix a vibration. the lift blocks could lead you to suspecting pinion angle is causing your vibration. Did you just buy this truck? Has it been vibrating as long as you've had it?

 

I bought the truck with the lift already on it. Had the truck for about 6 months already and it only started shaking like this about a month ago out of nowhere. I did grease my U-joints. Could it be overgreased?

 

Not likely, I would guess you have a bad u-joint or loose pinion or play in the slip joint since acceleration effects it.

 

Okay. Thank you so much for your help, I'm going to take a look at everything today and I will message you my outcome

 

i'm not positive, but if you have a center support bearing (Carrier) check that also.