when you drive my brothers 89 jeep cherokee at about 45 mph it starts to shake bad been told by a guy i know who drag races that its the tranny but i can drive in 5th without the wobble it just seems to be at about 45 mph that and i have smoke come out of the shifter boot sometimes i dont have accsess to the jeep so im limited on details just wondering if anyone has any ideas to lead me on a path to fixing it ive thought the smoke might be burning oil cause i belive i was told the engine leaks oil and the boots torn up i just want to figure out weather i should buy a whole new engine and tranny for it or fix it befor i buy it off him he bought it as a project and its been like that sense we got it so i have no clue how long this problems been there  

first things first, check the balance on those tires and shakedown the the frontend parts, and check the u joints on the rear driveshaft. pretty general answers but you will probably find the problem.  

the shake may be loose front end componets, the smoke...  oil burnin off the exhaust pipe.  

Also, check the motor mounts. They are often bad and can cause wierd shaking that people swear is the tires or front end shaking.  

thank you guys you'll probably hear more from me i can do most the labor i am just not good at trouble shooting yet not enough experience