Need help with diagnosing vibration problem YJ

It seems to have a mind of it's own. Some days the YJ runs like new, other days I could trade it in. Any how. Mine is a 5spd manual (see vehicle info below). Some times when I start out in 1st gear I get a shudder, as if the clutch is bad. And if you use reverse gear it's also real bad. But when your moving and get past the intial clutch engagement everything is real smooth? This has been going on and off for the last 8 years. I first thought it was the clutch. I noticed a oil leak on the bottom of the bell housing and had it check, the shop said it was a rear main seal. So I thought the clutch had a little oil on it. So I had the shop pull my clutch. Clutch disc was perfect, no oil at all and the t-brg wasn't leaking.. But sense the clutch assembly was out I had them replace with all new parts, clean the flywheel, replace the pilot brg, clutch disc, pressure plate and t-brg. No change at all. Everything looks good underneath, U-joints are all good, no slop in trans/transfer/diffs. Could it be a problem with the rear limited slip diff? I do service the rear diff every year and add the little tube of additive. It seems to be coming from the rear? :crying:

1991 Jeep Wrangle Renegade, A/c, Hard top
Engine 4.0 liters MFI 6 cyl 258 cubic inches
Transmission Manual 5 speed: AX15 ???
Transfer Case: NP231 ???
Front differential: Dana 30
Rear Differential Limited Slip: Dana 35, 355 gears
Tire Size: BFG AT 30x9.50 R15LT

Need help with diagnosing vibration problem YJ