Okay had my 373's put in and 323's taken out today. really noticed a pick up diff.

but from 70mph to 100mph it vibrates pretty good. can feel it up thru the console .. I don't think it is the gears, no noise or howling. just a rumble and vibration. not the tires feels like the driveshaft. could it became out of balance at the shop?
the gear installer seems to think it somehow is the d.s. I turned it 180' and put it back in, no difference it is original d.s. 147,000 miles. could it be that it is spinning faster maybe causing the d.s. to vibrate. going to try to find someone to balance it and put in new yokes hopefully tomorrow..any ideas...

 

Mine vibrated worse going from 3.23 to 3.73, but the vibe was there before the ratio change, with the factory set-up, just not as noticable.
I think trying to balance the original GM shaft may be a waist of time, but some say they had good results.
Do a search and you'll see plenty of pre 98 f-body owners installed either a 98+ aluminum DS or an aftermarket.
If you install a polyeurathane tranny mount or torque arm bushing, the vibe will be much worse {or much more noticable} in my experience.
You may do OK trying to correct that shaft, but many have tried with less than satisfactory results.
GM did release a "technical service bulliten" for this problem, but never a recall.
The TSB went like this:
DS vibeTSB 1993-96 f-body vin p
Propeller Shaft - Vibration at Highway Speed
File In Section: 4 - Drive Axle
Bulletin No.: 67-41-01
Date: March, 1996
Subject:
Vibration at Highway Speed (Replace Propeller Shaft)
Models:
1993-96 Chevrolet Camaro
1993-96 Pontiac Firebird
with 5.7L V8 Engine (VIN P - RPO LT1)
Condition
Some owners may experience a vehicle vibration at highway speeds. When measured at the driver's seat track, this vibration will be a 1st order driveline frequency. The same vibration can be confirmed at the axle pinion nose and at the rear extension housing of the transmission.

Important :When measuring vibration frequency on a hoist, the rear of the vehicle must be supported by the axle tubes in order to maintain the proper vehicle trim height. This will prevent vibration due to excessive driveline angle. Also, any vibration due to excessive driveline angle will appear as 2nd order driveline frequency vibration.
Cause
First order driveline vibration may be caused by beaming between the engine and transmission.
Correction
A previously released propeller shaft is available to address the highway speed vibration condition. This reduced mass propeller shaft is constructed of aluminum less a pinion damper.
Verify condition per above note that the vibration is 1st order driveline disturbance; preferably using an Electronic Vibration Analyzer (EVA).
Other conditions such as excessive driveline angle difference may produce 2nd order driveline vibration concerns. The proper procedures to measure and correct for excessive driveline angle condition are not shown in the current Service Manual. Consult a 1992 Service Manual for the proper procedure. The front and rear propeller shaft angle must be less than one (1°) degree of each other.

Important : Due to the removed damper, the alternate propeller shaft may unmask existing rear axle noise. A lighter propeller shaft will reduce vibration but will possibly unmask axle noise, while a heavier propeller shaft will mask axle noise but may cause vibration. This inherent condition is always the trade-off. It is suggested that the vehicle be test driven to evaluate axle noise before and after parts are installed.

Install alternate propeller shaft,
Part Number 10085375.
Warranty Information
For vehicles repaired under warranty, use:
Labor Operation Labor Time
F1280 Use published labor operation time.