95 2wd s10 vibration at certain speeds

Hello all, Ive been chasing this problem for awhile and i cant seem to be able to track it down so heres everything that is needed to know:

1995 Chevy s10 2wd 5 speed 2.2L 4 banger reg cab reg bed stock height 220K miles on it. has a single piece driveshaft with a u joint at trans and at pumpkin

Issue is at 45mph in 5th gear i get a vibration that i can hear/feel in the seat / floor / and shifter knob at cruising speed.

I can push in the clutch vibration goes away.

push in clutch, DONT let the rpm fall let clutch back out the vibration stays the same.

push in clutch and DO let the rpm fall to idle, rev back up let clutch out and vibration is either gone or changes in severity.

also sometimes does it while lightly accelerating at around 2K rpm in 2nd, and while engine braking in 3rd going 35mph SOMETIMES.

at 55mph+ its smooth

never feel it in the steering wheel

Things that have been done that didn’t change anything:

rims and tires were removed, sandblasted, painted, remounted and balanced – ran on factory rims and tires off my 02 blazer (same lug pattern) rode smoother due to more of a car tire tread but vibration was still there and came and went as it does.

checked and changed the trans fluid – no funny particles stuck to magnet or in bottom of drain pan.

rearend fluid has been checked and changed when axle seals leaked (before vibration)

new front wheel bearings

2 new rear u joints (one MOOG – not sure if i got it installed right – whole nother can of worms and me vowing never to use MOOG u joints again – replaced with different brand this time and pumped full of grease till the mystic jt6 synthetic blue came out all 4 caps – i prepumped all the factory anti rust grease out of the spider before installation).

clutch, throw out bearing, piolit bearing, flywheel pressure plate, slave cyl were all repalced.

^^^ all of this has been replaced since 208K miles due to it wearing out or trying to track down the vibration.

now heres some other things to know:

my own exhaust creation, factory header/downpipe to 2 1/4″ straight pipe from under the passengers feet area to just behind the bed to a cheapo dynomax turbo muffler out the stock exhaust up over the axle out at the stock location. it does cause soem drone and vibrations but its been that setup for years – ive checked it and its all how i had it last supported – not rubbing anything.

does have a fiberglass topper

rear springs are a bit sprung (been loaded up a bunch in its life) but truck still sits level – when i go over dips or bumps when its vibrating it doesnt seem to affect the vibration so i think pinion angle is still OK.

ive added the clamp on helper springs (single bar and U bolt setup) that kinda help but not really – i may take them off and see what happens

does have air shocks that i keep some air in to give it a smooth ride and when it does have a load to help it out some.

overload leafs have been removed due to one cracking in half at the bolt – new grade 8 bolts installed on both sides (hasnt been loaded heavy since and dont plan to)

front U joint is factory but is loose as a goose when drive-shaft has been out – i dont feel any binding at all (IF A U JOINT IS TOO LOOSE CAN IT CAUSE A VIBRATION?) i dont detect any play in it in left right up down or as if i were to twist it. but like i said it does have 220K miles on it…

drive shaft doesn’t appear to be bent or any dings in it, all weights appear to be there.

with hood open and throttle snapped the engine appears to be pretty solid (aka motor mounts OK)

in the morning (like morning commute) it doesnt seem to vibrate but sometimes will with the afternoon commute – consists of residential 25mph road, then a 40mph road then on ramp to 65mph interstate for 2 miles then off on 30mph side streets

Im usually pretty good with cars but this one has me stumped…