help,this truck is driving me nuts

[85BURBAN]

85 gmc jimmy,305/700r4/np208 the problem i have is this vibration in the driveline its intermidatent its fine for two minutes then vibrates like heck,then after a minute it good,etc. etc. now the guy i bought it from said it has 3/4 springs theres 5 leafs in the front and 7 in the back, are these really heavy duty or is this a lift kit? i put a 4 inch leaf in my last truck and the springs had alot more arc than these do.also it had 1 inch blocks in the rear.the pionon angle looked severe to me so i added 4 degree shims.the problem i have is the pins on the leafs didnt make it through the shims to the rearend so i thought maybe it was out of line.so i pulled the shims,removed the 1 inch block seated the rearend in the leafs and bolted it back together,and thats when the intermidant vibration started(instead of all the time) any answers would be much appreciated,suggestions,anything, oh is it possible to remove leafs to help bring the rearend closer to the frame and reduce the pinion angle if thats what my problems is,again any input is much appreciated the wife already telling me "i told you to trade in that clunker" but God help me and my love 73-87 gm trucks and suv's

[joesgarage71]

If we all listened to our wives we'd be driving mini vans and volvos.

 Check driveshafts, all u-joints and CV's, motor mounts , trany mount, transfur case mounts, ball joints,and tierod ends. Oh leaf spring bushings and drag link.
If you have the slip yoke 208 then remove the rear driveshaft and in 4WD drive it to see if there is still the problem, if not then I'm not sure there might be a bushing that is inside the rear of the transfur case that rides on the outside of the slip yoke of your driveshaft,( if your getting my meaning ) that is wore out( my TH400 had a bad one I replaced) It will let the slip yoke wobble and cause your vibration.
 

[85BURBAN]

driveshafts new ,new u joints,tranfer and tranny mounts all seem good my mechanic who does what i dont have the time or pateince to do and does all my wifes cars work(99 grand am gt)cause under that hood scares the crap out of me ,thinks its the pinon angle and he saw and drove it with the shims in it today said it definately something in the rear jimmys never came with heavy duty as far as i know and they ad alot more height than my burbans factory 5 rear leafs which had 4 inch blocks in it and the pionon angle wasnt nearly as heavy as the jimmy obviosly longer driveshaft but and this point i would rather pull leafs out (if possible)just to see if it makes a difference. it was fine for the first 6 months i had it, it strated really vibrating on the highway i pulled off it got worse as i drove then the driveshaft finally let go luckly 500 yards from a friends house and it hasnt really been right since i replaced the driveshaft,even with the new u-joints

[joesgarage71]

driveshaft is new, is the slip yoke new?

[85BURBAN]

the bushing your refering to is it the black seal on the output of the transfer your talking about,where the driveshaft goes in?

[85BURBAN]

i should ass that the problems is not right away it useually takes a minute or to to start vibrating, my brother in law seems to think it might be the rearend somewhere,he gave me a few places to start,things to check,his idea was the driveshaft broke once at the rearend what caused that and is it ready to happen again?

[85BURBAN]

i should add that ....sorry for the dumb typo

[Marc]

You would want to make sure your u-joints aren't binding...I had a vibration in my truck and one of the u-joints was stiff - had been assembled too tight.  Took it apart and ground the rust off the inside of the yoke that the clip sits against and it was much better.

Other thing I had happen to me  - I have a 71 Scout that had a bad cyclical vibration like you describe.  If the driveshaft on the Jimmy is like the one on the Scout it has a slip yoke and is not keyed.  It was assembled at some point out of phase - the universals have to line up perfectly or it will vibrate badly...if you can visualise it the universals have to be lined up like + on one end and + on the other, or X on one end, or X on the other...it can't be a + on one end and a X on the other. 

Is the pinion loose in the rear end?

[85BURBAN]

yeah the driveshaft yoke is keyed i know for sure cause when i first put it in it vibrated immediately,spun it was 180 and it was good,not not perfect,as for the pinion,my bro-in-law is on his way (mechanic by trade,and 73-87 gm lover) to see whats going on the problem seems to be resent,it wasnt bad for the last month or so in the last 3 days it become unnerving,it starts out fine for the first few minutes of driving then starts to vibrate where the change in the ashtray rattles,but if i let off of the gas its smooth so it only when the drivetrain it put under pressure.

[Marc]

Almost sounds like a miss in the engine...

[85BURBAN]

thank God my little sister is good for something....she married a smart gm guy after a couple of test drives and tinkering my bro-in law found the problem one of the c clips on the u-joint at the rearend was smaller then the other three luckily thats one of the extra parts i drive around with(just in case) installed the clip the driveshaft and it run like...well it runs like a 24 year ols new england truck,and thats all i really wanted,thanks to everyone for there input as always this sight and its members have been extra helpful thank you all
                                                         ray
 
                                                         

[joesgarage71]

the bushing your refering to is it the black seal on the output of the transfer your talking about,where the driveshaft goes in?

The bushing is inside that rubber seal, in the body of the tailshaft.