New guy needs help..... Bad!!!

Have a 87 sport, it was purchased new in 87 by the usaf and used until 97 when Fort Gaines Police Dept got it and kept it till I got it. Truck runs like a champ but vibrates bad at high speed. It started at 45 so I had tires rotated and balanced and guy said they all balanced great. Changed trans mount with new not china made and both u joints. Had to use 4 wd u joints for some reason. Replaced idler arm bushing also. All steering and suspension seems solid but I know looks can be deceiving but seems to have no play. I did notice my torsion bar bushing I believe it's called are cracked up bad. Trans still clunks in gear too but not all the time and when driveshaft was taken out fluid was light brownish color no red tint. Any suggestions what to look at next. I'm wondering if my steel wheels are centering correctly or maybe unknown bushings wore out. Should I just shoot to rebuild the entire frontend. How can I tell if my output shaft in trans is warped. I'm just at a loss as to the cause of this vibration but will not give up or in until solution is found as I have to drive to work and need to do 70 not 45 on interstate. Lol

The driveline center bearing is bad. The rubber allows the conector shaft to drop and then the driveline is out of spec on the u joints. Rebuilding the drive line will fix it.
as for the oil in the trans. Just change it and the filter.

Make sure you get the right carrier bearing for the drive line. There are a few around and some are close but not exact. I bought one that resulted in needing a pressed on cup/cover removed from a yoke to get it to fit without the bearing getting jammed up.

I installed both new u joints and trans mount and idler arm bushing and rotate and balance.

I have no center carrier bearing.

A few questions for you: Does the steering wheel shake when you feel the vibration? Does it feel like it's coming from the front, rear or center of the truck? When present, does the vibration increase, decrease or remain unchanged when accelerating, decelerating or under only light load? Did you have a shop replace the u-joints?

I can't feel it in the steering really and it seems present all the time but I can't go over 50 or it feels it gonna shake apart. Doing lca bushing and strut rod bushings this weekend.

could it possibly be the wheel hub/bearing? stupid question i know. does the truck become hard to control? does more speed help the vibration a little?

I call drive shaft needs to be re-balanced...