Changing clutch

[Just me]

Hello all. I'm changing my clutch this week but I wanted to make sure it was the clutch one last time. Thanks for answering my post before. Since this is still fresh in my mind, I took the car out this morning and while it was cold had no problems bringing the car out of 1st gear. I didn't even have the poblem the whole 20 minutes of driving. I went to the teller then drove again and still no problem. I went to eat some fast food then got back into the car and this is where my problem starts. Engaging into 1st gear from a stop gives me bad chatter and the whole car shakes, but only 1st gear. So I've decided to change my clutch this week. I changed the clutch just 2 years ago and haven't had any slipping. Do any of you think it may be something else? I just wanted to make sure. How would I know if its the flywheel or pressure plate? Also should I change my rear main seal? If so, would I need any special tools? Thanks in advance.

Just me
91'lx, 302 block, crank pulley, no smog, short belt, Wolverine 1087 cam, 15 deg timing, TFS TW heads, motorsport 1.6 3/8 stud RR, ARP studs, TFS chrome hardened push rods, 24lb inj, BBK 255lph fuel pump, Areomotive billet adj FPR, Performer RPM intake(ported to 75mm), 1" spacer, Motorsport polished tall valve covers(love em), Pro-M 77, Acufab 75mm TB, Ford idle air bypass valve, K&N conical, MSD 6AL and coil, 9mm MS cables, NGK bkr5e 11 plugs@55, motorsport 3 core radiator,160 thermostat, Hurst short throw, motorsport 410 gears, aluminum drive shaft,Kenny brown Subs, Mac 1 3/4"Long tube, custom 3"off road xpipe, 2 chamber Flow Masters, 3 1/2" tailpipes HAL tubular K-member, KYB struts and shocks, Eibach springs, Steeda 2.5" Cowl, Steeda rear wing, black Dupont centari acrylic enamel paint.