PLEASE HELP!!....where to start??
 

ok guys try and make this as short as i can.
got 2001 cummins 5.9 24v, truck was in accident 3 years ago 10g's in damage inssurance fixed it at a very reliable and family friend body shop, Viel's auto body, they put a good "used" diff in the back complet assem. never had any problems with truck till now, i travel about 200km away for work and then from camp to work about 25-30km in a 4 day week i put about 500-600km, so i blew the rear diff well witch we thought it was so called scrap yard out of town and got a dana 70 with 40,xxxkm for 1750$ but would not be in for 2 weeks, so me and a buddy of mine took the old diff from the body shop, body shop said one of the tubes were bent, so went and digged it out behind the garage and installed carrier in blow diff in truck ( we kept it as a set BTW) but with no tool for pre load nor to check back lash we went by feel so drove truck for 2 weeks like that witch made little bit of noise, finaly install new diff replaced u joint on 4x4 shaft the one closer to the axle and replaced trans mount witch was broken in half and broken in every way possible, so after all this this truck has a vibration, you feel it in the steering wheel a little, but mostly in floor board and seats dunno where to start, i do need to replace front wheel bearing on pass and outer tie rod but just want some input please

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also forgot to mention it seems to be shifting erraticly in and out of lock down, also RPM's seem to play at steady speed up and down a little this all occures at 90-100km u feel iit lots not as much in lower speeds

May not be the same, but my 93 rear dif locked up, replaced it and and had vibration , had to replace the tranny. I guess it stressed when the dif stopped turning, any way something bent just enough. Good luck

how can you tell difference between intercooled or not in my 1991 dodge cummins

Your intake would go to an intercooler in front of your truck then back over to the other side.

Now the rpm lock up is that your TC is locking and unlocking my truck did this check all your connections on your batts a loose connection will mess with the TC or even a weak batt.

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Wheel bearings are a pain in the rear takes about 2 hours or less to do one you have to remove the wheel, then caliber. Next is the axel nut which is a 1 15/16 socket I belive. There are four bolts on the back of the wheel assembly you have to remove then take a 10 inch extension for a socket or something similar and then but it behind the wheel hub and against your lower controll arm start the truck and turn the wheels and it will pop it off. Then you have to pound out all the studs and drive them back in with the rotor.