Are your engine mounts ok, they are often shagged and it's hard to tell without removing them.
clutch or flywheel out of balance (if these have been done recently)
suspension bushes shot
as you said diff mounts might be shot, or any mounts affecting the rotating assembly
are all wheels the correct centre bore (be it with spigot rings or just as they are)?
all suspension bolts done up correctly?
quality brake rotors (if these have been changed recently) as some cheap ebay ones can be unbalanced
UKDM 1998 1.8 Mk2 Sport - Current mods- Custom 6 point chassis mounted front splitter and sideskirt extensions, battery cut off switch, OEM 'small skirts' and lip, HEL braided brake lines, OEM front upper strut brace with MC brace and mazdaspeed sticker, custom made 2" high flow catback exhaust, Japspeed 4-1 exhaust manifold, Lorber 15" alloys in fiat 750 maroon volcano, uniroyal rainsport2s, Ebay special JDMyo! gear knob, tinted indicators, colour coded reflectors, black headlight inserts, LED sidelights, SMD number plate bulbs, dimpled and grooved mtec brake discs, mintex brake pads, de-tango'd headlights, amber bulbs, RGB footwell lights, SMD interior light, Deltastyling spoiler, foamectomy seats, mazdaspeed seat belt pads, ring ultimax headlight bulbs, IL Motorsport style bar in custom paint, ford fan washer jets, Skidnation coolant reroute with Kia thermostat housing, Gates 22436 radiator hose with hose bracket, 82/85 degree twin valve thermostat, D1 spec engine bay dress bolts, stainless steel bolts throughout engine bay, EGR blanking kit, painted brake calipers, painted tow hooks front and rear, wrinkle painted cam cover, de-greened dials with white SMDs, red needles, painted cigarette lighter, painted oil filler cap, custom made cup holder, painted radiator retainers, polished slam panel, painted alternator, painted upper manifold, painted throttle body, painted header heat shield, painted relay bracket, painted loom brackets, painted diagnostics box bracket, painted wash fluid brackets, painted ABS brackets, painted airbox brackets, painted horns and brackets, painted throttle cable brackets, painted charcoal canister brackets and hard piping, painted wiper rods, painted engine hooks, painted clutch line bracket, Jass offset license plate bracket, offset square plate, 1lt foam fire extinguisher on custom bracket and custom Velcro retaining strap, POR15 rust proofed to within an inch of it's life, cavity waxed and fully seam sealed
UKDM 2001 mk2.5 - Current mods- SMD interior light, SMD number plate lights, Corbeau Sprint (yes it fits!), modified mk2.5 door card, Skidnation seat brackets, Corba 1046 remote central locking unit, Driftworks basics 350mm leather steering wheel, NRG short hub, NRG 2.5 quick release hub, 8" weighted gear knob, factory electric wing mirrors, factory electric aerial, Kage 4" exhaust with additional powerflow silencer, foam fire extinguisher, custom fire extinguisher mounting bracket, painted cam cover, painted oil filer cap, 15" Advan SA3Rs, Nankang NS-2R 180tw, offset plate bracket, RGB footwell lighting, sun strip, painted calipers and carriers, painted headlight inserts, softtop delete, brake fluid reservoir sock, GCR3 roll bar in custom gloss black, Corbeau Ultima Pro 2006SSU 6 point harness, push button bumper quick release pins, twin port oil catch can, engine bay respray, fully POR15 rust proofed chassis, full chassis seam seal, custom permanent hardtop side mounts, Jass permanent hardtop front mounts, M-tuned large rear rotor spacers with corresponding 276mm SVT brake discs, Freakyparts larger front rotor spacers with corresponding 11" Corado G60 front rotors, HEL braided brake lines, Byba Kits GV style front lip, SVT under chassis bracing, painted radiator brackets, mazdaspeed seat belt pads, charcoal canister delete, large Mazdaspeed side skirts, adapted NA short shifter, 60mm alloy radiator, 3" brake ducts, upper strut brace with MC brace, 3.9 type 2 torsen, 80A diff bushes, front tow hook, rear tow hook
2004 Celica T Sport - Current mods- Meister Zeta Pro coilovers, Forged Wilwood dynalite front calipers, Yellow stuff brake pads up front, Drimpled and slotted discs, Braided brake lines, TRD stubby aerial, Short shift kit, 17" alloys powdercoated in metallic granite, TRD centre caps, Invidia exhaust, Cusco brake master cylinder brace, SMD side light bulbs, butterfly valve delete, Aux to CD converter, 8" gear knob, sun strip, interior SMD bulb, remote damper adjusters
Sold- UKDM 1992 1.6 Mk1 SE - Current mods- Yokohama A048 semi slicks, 15" Lorber 7JJ deep dish alloys, Tein street springs on OEM dampers, IL Motorsport catback exhaust, IL Motorsport decat, K&N greenstuff filter, OMP 330mm racing wheel, TR Lane half cage, TR Lane harness bar, Elise bucket seats, Sparco 4 point harness, Ebay special JDMyo! gear knob, GV lip, Vindii ducktail